ectoparasite fitness in auxiliary hosts: phylogenetic distance from a principal host matters

Ectoparasite fitness in auxiliary hosts: phylogenetic distancefrom a principal host matters

I . S . KHOKHLOVA* , L . J . F IELDEN† , A . A . DEGEN* & B. R. KRASNOV‡*Wyler Department of Dryland Agriculture, French Associates Institute for Agriculture and Biotechnology of Drylands, Jacob Blaustein Institutes for Desert Research,

Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel

†School of Science and Math, Truman State University, Kirksville, MO, USA

‡Mitrani Department of Desert Ecology, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion

University of the Negev, Midreshet Ben-Gurion, Israel

Keywords:

auxiliary hosts;

fitness;

fleas;

principal host;

rodents.

Abstract

We studied reproductive performance in two flea species (Parapulex chephre-

nis and Xenopsylla ramesis) exploiting either a principal or one of eight auxil-

iary host species. We predicted that fleas would produce more eggs and

adult offspring when exploiting (i) a principal host than an auxiliary host

and (ii) an auxiliary host phylogenetically close to a principal host than an

auxiliary host phylogenetically distant from a principal host. In both flea

species, egg production per female after one feeding and production of new

imago after a timed period of an uninterrupted stay on a host differed signif-

icantly between host species. In general, egg and/or new imago production

in fleas feeding on an auxiliary host was lower than in fleas feeding on the

principal host, except for the auxiliary host that was the closest relative of

the principal host. When all auxiliary host species were considered, we did

not find any significant relationship between either egg or new imago pro-

duction in fleas exploiting an auxiliary host and phylogenetic distance

between this host and the principal host. However, when the analyses were

restricted to auxiliary hosts belonging to the same family as the principal

host (Muridae), new imago production (for P. chephrenis) or both egg and

new imago production (for X. ramesis) in an auxiliary host decreased signifi-

cantly with an increase in phylogenetic distance between the auxiliary and

principal host. Our results demonstrated that a parasite achieves higher fit-

ness in auxiliary hosts that are either the most closely related to or the most

distant from its principal host. This may affect host associations of a parasite

invading new areas.

Introduction

One of the main mechanisms to maximize reproductive

success of an individual is selection of habitats that pro-

vide the greatest fitness output (Rosenzweig, 1981,

1987, 1991). Selection of a habitat with higher per cap-

ita resource abundance and/or that allows easier

resource acquisition presumably results in higher fitness

reward. One of the consequences of fitness increase is

an increased abundance, so that the abundance of a

consumer in a habitat is often considered a measure of

its performance and efficiency of resource exploitation

in this habitat (Morris, 1987).

Long before the theory of habitat selection was for-

mulated, scientists who studied parasitic species imple-

mented similar ideas for the explanation of unequal

distribution of conspecific parasites among different

host species. Traditionally, a host in which a parasite

attains the highest abundance and/or prevalence is con-

sidered to be its principal host, whereas other hosts

exploited by the parasite are considered to be auxiliary

hosts (Dogiel et al., 1961; Dogiel, 1964). It has been

Correspondence: Boris R. Krasnov, Mitrani Department of Desert Ecology,

Swiss Institute for Dryland Environmental and Energy Research, Jacob

Blaustein Institutes for Desert Research, Ben-Gurion University of the

Negev, Sede-Boqer Campus, 84990 Midreshet Ben-Gurion, Israel.

Tel.: +972 8 6596841; fax: +972 8 6596772 e-mail: [email protected]

ª 2 01 2 THE AUTHORS . J . E VOL . B I OL .

1JOURNAL OF EVOLUT IONARY B IO LOGY ª 20 1 2 EUROPEAN SOC I E TY FOR EVOLUT IONARY B IO LOGY

doi: 10.1111/j.1420-9101.2012.02577.x

shown that, in at least some parasites, exploitation of

the principal versus the auxiliary host has resulted in

different fitness rewards with the rewards being highest

with the principal host (Krasnov et al., 2002).

Nevertheless, abundance and/or prevalence of a para-

site varies substantially among auxiliary hosts. To

account for this variation, parasitologists classify an

auxiliary host as being a ‘normal’, ‘secondary’, ‘acci-

dental’ or ‘exceptional’ host for a parasite (Hopkins,

1949; Holland, 1964; Wenzel & Tipton, 1966; Marshall,

1981). Mechanisms for the variation in abundance of a

parasite among auxiliary host are poorly understood.

Poulin (2005) proposed a hypothesis that the ultimate

cause for the variation in parasite abundance among

auxiliary hosts is the difference among these hosts in

their phylogenetic relatedness to the principal host. The

rationale underlying this hypothesis is that phyloge-

netic relatedness among species should mirror their

physiological and ecological similarities (e.g. Harvey &

Pagel, 1991). If so, then an increase in phylogenetic dis-

tance between an auxiliary host and a principal host

should correlate negatively with parasite abundance in

the auxiliary host. Such a negative correlation was

reported for fleas (Insecta: Siphonaptera) parasitic on

small Holarctic mammals (Krasnov et al., 2004a); how-

ever, Poulin (2005) did not find any support for this

prediction for metazoan parasites of freshwater fish.

In an attempt to find proximate mechanisms for the

decrease in flea abundance in auxiliary hosts with an

increase in phylogenetic distance from the principal

hosts, Khokhlova et al. (2012a,b) investigated feeding

performance of two flea species (Xenopsylla ramesis and

Parapulex chephrenis) when they exploited their princi-

pal hosts and auxiliary hosts with varying phylogenetic

relatedness to the principal hosts. Feeding performance

of fleas was measured by bloodmeal size and energy

expenditure for blood digestion. Unexpectedly, it was

found that fleas did not always perform better on a

principal host than on an auxiliary host and that, in

some cases, fleas fed better on hosts that were phyloge-

netically distant from than close to their principal host.

One of the reasons behind the contradiction between

these results and the pattern of flea abundances in aux-

iliary hosts with different degrees of relatedness to the

principal host is that variation in feeding performance

(including energy cost of bloodmeal processing) among

hosts might not necessarily be a good proxy for fitness

achieved in these hosts. Nevertheless, haematological

properties vary between vertebrate species (Harrington

et al., 2001) and therefore could affect the fitness of

haematophagous insects via their effect on the nutritive

and/or energy value of blood (Harrington et al., 2001;

Lyimo & Ferguson, 2009; Lyimo et al., 2012). Conse-

quently, identifying the proximate mechanism that

explains variation in parasite abundance among auxil-

iary hosts as a function of phylogenetic distances

between these hosts and the principal host requires

experimental measurement of direct fitness-related

variables that would measure the ability of a parasite to

translate resources extracted from a host into the

parasite’s offspring.

Here, we addressed the question of the effect of phy-

logenetic distance between an auxiliary host and princi-

pal host on reproductive performance of fleas in this

auxiliary host. To answer this question, we measured

fitness via production of eggs and new adults in two

flea species (Parapulex chephrenis and Xenopsylla ramesis)

after they exploited either a principal or one of eight

auxiliary host species. We predicted that fleas would

produce more eggs and more new adults when exploit-

ing (i) a principal host than an auxiliary host and (ii)

an auxiliary host phylogenetically close to a principal

host than one phylogenetically distant from a principal

host. The two flea species used in this study are com-

mon in desert habitats of southern Israel. Parapulex che-

phrenis is parasitic mainly on the Egyptian spiny mouse

Acomys cahirinus and is found less often on the golden

spiny mouse Acomys russatus and the gerbils Meriones

crassus and Gerbillus dasyurus (Krasnov et al., 1997,

1999). Xenopsylla ramesis is found on a variety of gerbil-

line rodents, but attains the highest abundance and

prevalence on M. crassus (Krasnov et al., 1997, 1999).

Materials and methods

Fleas and rodents

We used fleas from our laboratory colonies started in

1999 from field-collected specimens. Parapulex chephre-

nis were collected from A. cahirinus, whereas X. ramesis

were collected from Psammomys obesus, M. crassus and

G. dasyurus. Details on breeding and maintenance of

fleas can be found elsewhere (e.g. Krasnov et al., 2002,

2003; Khokhlova et al., 2009a,b, 2010a). In brief, fleas

were maintained on their characteristic rodent hosts

(P. chephrenis on A. cahirinus and A. russatus and X. ra-

mesis on M. crassus and G. dasyurus) that were kept indi-

vidually in plastic cages with a wire mesh floor over a

pan with a mixture of sand and dried bovine blood (lar-

val nutrient medium). Air temperature was maintained

at 25 °C and photoperiod at 12:12 (L:D) h. Every

2 weeks, all substrate and bedding material from each

rodent cage (including nest box and pan) were col-

lected into plastic boxes with perforated lids and trans-

ferred to an incubator (FOC225E; Velp Scientifica srl,

Milano, Italy; 25 °C air temperature and 75% relative

humidity) where the fleas developed. All fleas used in

experiments were selected randomly from colonies.

We used nine rodent species, eight of them from the

family Muridae and one from the family Cricetidae

(Mesocricetus auratus). Among murids, there were five

gerbillines (M. crassus, G. dasyurus, Gerbillus andersoni,

Gerbillus pyramidum and Gerbillus nanus) and three

murines (A. cahirinus, A. russatus and Mus musculus).

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2 I . S . KHOKHLOVA ET AL.

Meriones crassus, G. dasyurus, G. nanus and both Acomys

spp. were from laboratory colonies started in 1997–1999 and 2009 (G. nanus). Gerbillus andersoni, G. pyrami-

dum and M. musculus (feral house mice) were captured

in the wild in desert habitats (gerbils) or settlements

(mice) in southern Israel. The rodents had all ectopara-

sites removed using a toothbrush and forceps and then

were maintained in a separate animal room for

2 months prior to experiments. Mesocricetus auratus

(golden hamsters) were available commercially.

Details on rodent maintenance in colonies can be

found elsewhere (Krasnov et al., 2002, 2003; Khokhl-

ova et al., 2009a,b, 2010a). In short, rodents were

maintained in plastic cages (60 9 50 cm and 40 cm

high) with sawdust bedding at 25 °C air temperature

and 12D:12L regime. Initially, each cage contained a

male and a female. Young individuals were moved to a

new cage at 2 months of age to prevent overpopulation

and inbreeding. Millet seeds and alfalfa (Medicago sp.)

were offered daily ad libitum. No drinking water was

available to the rodents as the alfalfa supplied enough

moisture for their needs. Spiny mice (Acomys) were also

offered commercial cat chow once a week. Only male

rodents aged 6–8 months were used in experiments.

Each rodent was used for feeding a single group of fleas

only once.

Experimental procedures

Fleas were randomly selected from the incubated devel-

opment boxes. An individual rodent was placed in a

plastic cage (60 cm by 50 cm by 40 cm) with a floor of

3–5 mm of clean sand covered by a wire mesh (5 mm

by 5 mm). According to a rodent’s size, 20–50 newly

emerged (24- to 48-h-old) female and 10–30 male fleas

(P. chephrenis or X. ramesis) were released into a cage

and allowed to feed for 3 days. Our preliminary observa-

tions demonstrated that fleas start to oviposit no sooner

than on the second day of the stay on a host under these

conditions. Each treatment with each flea species and

each host species was replicated five to nine times.

After 3 days of an uninterrupted stay in a rodent’s

cage, fleas were collected from both the rodent’s body

and cage substrate. To collect fleas from the rodent’s

body, we brushed them out over a white plastic pan

with a toothbrush. The hair of the rodent was brushed

several times until no flea was recovered. We examined

the fleas collected from the rodent’s body and cage sub-

strate under light microscopy (409 magnification) and

counted the number of fleas (males and females sepa-

rately) with and without blood in their midguts. After

collecting fleas, the substrate was removed from each

cage, added to about 0.5 g of larvae nutrient medium

(94% dry bovine blood, 5% millet flour and 1% grin-

ded excrements of the respective host species), placed

in a plastic box (20 cm by 10 cm by 10 cm) and trans-

ferred to an incubator at 25 °C air temperature and

92–95% RH. Relative humidity was regulated in 38 cm

by 23 cm by 13 cm acrylic humidity chambers using

saturated salt solutions. Temperature and humidity

were monitored using Fisherbrand Traceable Humidity/

Temperature Pen with Memory (Fisher Scientific Inter-

national, NJ, USA). Starting on the 35th or 24th day

after the onset of the experiments (minimal duration of

development of P. chephrenis and X. ramesis, respec-

tively, established in our earlier experiments; see Kras-

nov et al., 2001; Khokhlova et al., 2010b), we checked

the boxes daily until all new adults emerged or for

2 weeks after the emergence of the last adult. We

counted newly emerged fleas and calculated offspring

production in each group of fleas per parent female flea

that took a blood meal during the 3-day uninterrupted

stay on a host. Because some fleas were killed by a

rodent during this period, the number of parent females

that produced eggs was estimated as an average

between the number of female fleas placed on a rodent

(i.e. initial number of females) and collected from this

rodent and cage substrate (i.e. final number of females)

after 3 days. Prior to this, the number of unfed females

(i.e. those with empty midguts) was subtracted from

both initial and final numbers of females. Re-analyses

of the data using either initial or final number of

females (with or without subtraction of the number of

unfed fleas) for calculating the offspring production did

not affect the results.

To obtain flea eggs, fleas of each group (i.e. recovered

from the same rodent individual) that took a bloodmeal

were placed in Petri dishes with the bottom covered by

a thin layer of clean sand and small pieces of filter

paper and were then transferred to an incubator

(FOC225E; Velp Scientifica srl, Milano, Italy) at 25 °Cair temperature and 90% RH for 24 h. On the fourth

day of the experiment, we checked the Petri dishes and

filter paper and collected all eggs laid by all females in

a given group. These eggs were not taken into account

in the subsequent analyses. On the fifth day of the

experiment, we placed each individual rodent in a wire

mesh (5 mm by 5 mm) tube (15 cm length and 5 cm

diameter for M. crassus, spiny mice and hamsters or

10 cm in length and 2 cm in diameter for other gerbils

and house mice) that limited movement and did not

allow self-grooming. Tubes with rodents were placed in

individual white plastic baths. Fleas (P. chephrenis or

X. ramesis) previously collected from this same rodent

were then released into the hair of the rodent. After

feeding on a host for 2 (X. ramesis) or 6 (P. chephrenis)

h (time necessary for satiation estimated from our preli-

minary observations), fleas were collected and exam-

ined for blood in the midguts as described above. Fleas

of each group with blood in their midguts were

returned to Petri dishes that contained sand and pieces

of filter paper and transferred to an incubator for 24 h,

after which we again checked the Petri dishes and filter

paper, counted newly laid eggs and calculated the mean

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Ectoparasite fitness in auxiliary hosts 3

number of eggs produced per fed female after one feed-

ing event.

Data analyses

Parapulex chephrenis and X. ramesis differ in their pat-

terns of host exploitation (time required for satiation,

frequency of feedings, time spend on and off the host

body; see Krasnov, 2008). Consequently, we analysed

data separately for each flea species. As mentioned

above, flea fitness was evaluated from the (a) mean

number of eggs produced by a fed female flea during a

day after one feeding event and (b) mean number of

new adults produced per parent female flea during

3 days of an uninterrupted stay on a host. Prior to

analyses, these dependent variables were log + 1-trans-

formed. Bar diagrams represent nontransformed data.

We used one-way ANOVAs to test for the effect of host

species on the fitness-related variables. To compare

mean egg production and mean production of new off-

spring between the principal host and each of the aux-

iliary hosts, we used univariate tests of significance for

planned comparisons. To test for the relationships

between the fitness-related variables of fleas when they

exploited an auxiliary host and the phylogenetic dis-

tance between this host and a principal host of a flea,

we calculated mean values of these variables across all

flea groups fed on the same auxiliary host species and

regressed them against the phylogenetic distance of an

auxiliary host from a principal host (A. cahirinus for

P. chephrenis and M. crassus for X. ramesis). Information

on topology and branch length of a phylogenetic tree

for the nine rodent species was obtained from various

sources (see details and the tree in Khokhlova et al.,

2012a). Phylogenetic distances were calculated from

branch length of a phylogenetic tree using the package

‘ape’ (Paradis et al., 2004) implemented in the R 2.13.0

software environment (R Development Core Team,

2011). As phylogenetic information was included in the

independent variable, no further phylogenetic correc-

tion was necessary.

Results

Egg production per female after one feeding differed

significantly among host species in both P. chephrenis

(F8,51 = 5.88, P < 0.001) and X. ramesis (F8,62 = 17.36,

P < 0.001). Parapulex chephrenis laid significantly more

eggs when they fed on Acomys hosts, M. crassus and

M. auratus, than when they fed on Gerbillus hosts

(Table 1, Fig. 1a). No eggs were produced by these fleas

after feeding on either M. musculus or G. nanus. Xenopsylla

ramesis produced the highest number of eggs when

feeding on M. crassus, G. andersoni and G. pyramidum,

Table 1 Univariate tests of significance for planned comparison of

egg production per female Parapulex chephrenis after one feeding

and new imago production after 3 days of uninterrupted feeding

on either the principal host (Acomys cahirinus) or one of eight

auxiliary hosts. Host rank reflects phylogenetic distance between

Acomys cahirinus and the auxiliary host.

Auxiliary host Rank

Egg production

F

New imago production

F

Acomys russatus 1 0.001ns 19.47**

Mus musculus 2 3.81* 50.45**

Gerbillus andersoni 3 13.54** 41.44**

Gerbillus dasyurus 4 12.50** 21.05**

Meriones crassus 5 0.10ns 16.65**

Gerbillus nanus 6 11.73** 46.29**

Gerbillus pyramidum 7 9.10** 34.93**

Mesocricetus auratus 8 0.002ns 4.01*

*P < 0.05, **P < 0.01, ns, nonsignificant.

Host species

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Mea

n nu

mbe

r of e

ggs

per f

emal

e (a)

Ac Ar Mm Ga Gd Mc Gn Gp Ma

Mc Ga Gd Gn Gp Ac Ar Mm Ma

Host species

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Mea

n nu

mbe

r of e

ggs

per f

emal

e (b)

Fig. 1 Egg production per female Parapulex chephrenis (a) or

Xenopsylla ramesis (b) after one feeding on either the principal host

or one of eight auxiliary hosts. Abbreviations of host species

names are as follows: Ac – Acomys cahirinus, Ar – Acomys russatus,

Ga – Gerbillus andersoni, Gd – Gerbillus dasyurus, Gn – Gerbillus

nanus, Gp – Gerbillus pyramidum, Ma – Mesocricetus auratus,

Mc – Meriones crassus, Mm – Mus musculus. Host species are ordered

according to their phylogenetic distance from the principal host

(Acomys cahirinus for Parapulex chephrenis and Meriones crassus for

Xenopsylla ramesis).




significantly less eggs when exploiting G. dasyurus,

G. nanus and M. auratus and extremely low numbers of

eggs after feeding on the three murine hosts (Table 2,

Fig. 1b).

Number of offspring produced by P. chephrenis that

fed continuously on their hosts for 3 days depended on

host species (F8,71 = 29.26, P < 0.001). This number

was (i) significantly higher for A. russatus than for

A. cahirinus and (ii) significantly higher for both Acomys

species than for the remaining hosts (Table 1, Fig. 2a).

The effect of host species on the production of new

imago by X. ramesis was also significant (F8,62 = 11.22,

P < 0.001) as this flea produced more offspring when

feeding on M. crassus, G. andersoni, G. pyramidum and

M. auratus than on other hosts (Table 2, Fig. 2b).

We did not find any significant relationships between

either egg or new imago production of either P. chephre-

nis or X. ramesis exploiting an auxiliary host and phylo-

genetic distance between this host and the principal

host (Table 3). However, when the analyses were

restricted to hosts belonging to the same family as the

principal host (Muridae; that is, when the cricetid

M. auratus was omitted from the analyses), new imago

production (for P. chephrenis) or both egg and new

imago production (for X. ramesis) in an auxiliary host

decreased significantly with an increase in phylogenetic

distance between an auxiliary and principal host

(Table 3; see illustrative example with new imago pro-

duction in Fig. 3).

Discussion

The results of this study partly supported our predictions

that fleas will produce more eggs and more new adults

when exploiting (i) a principal host than any auxiliary

host and (ii) an auxiliary host phylogenetically close to a

principal host than an auxiliary host phylogenetically

distant from a principal host. In general, egg and/or new

Table 2 Univariate tests of significance for planned comparison of

egg production per female Xenopsylla ramesis after one feeding and

new imago production after 3 days of uninterrupted feeding on

either the principal host (Meriones crassus) or one of eight auxiliary

hosts. Host rank reflects phylogenetic distance between Meriones

crassus and the auxiliary host.

Auxiliary host Rank

Egg production

F

New imago production

F

Gerbillus andersoni 1 0.13ns 1.38ns

Gerbillus dasyurus 2 5.57* 7.76**

Gerbillus nanus 3 27.58** 9.30**

Gerbillus pyramidum 4 1.42ns 2.42ns

Acomys cahirinus 5 46.36** 39.12**

Acomys russatus 6 48.65** 35.37**

Mus musculus 7 51.03** 39.12**

Mesocricetus auratus 8 3.89* 3.27ns


Host species

0

1

2

3

4

5

6

New

imag

o pr

oduc

ed p

er p

aren

t fem

ale

(a)

Ac Ar Mm Ga Gd Mc Gn Gp Ma

Mc Ga Gd Gn Gp Ac Ar Mm Ma

Host species

0

1

2

3

4

5

6

New

imag

o pr

oduc

ed p

er p

aren

t fem

ale

(b)

Fig. 2 Mean number of new imago produced per parent female

Parapulex chephrenis (a) or Xenopsylla ramesis (b) after 3 days of

uninterrupted feeding on either the principal host or one of eight

auxiliary hosts. See Fig. 1 for the abbreviations of host species

names. Host species are ordered according to their phylogenetic

distance from the principal host (Acomys cahirinus for Parapulex

chephrenis and Meriones crassus for Xenopsylla ramesis).

Table 3 Summary of regression analyses of egg (EP) and new

imago (IP) production of Parapulex chephrenis and Xenopsylla ramesis

when exploiting an auxiliary host and phylogenetic distance

between an auxiliary and the principal host.

Host

family in

analysis Flea

Dependent

variable r2 F Slope ± SE

Muridae

and

Cricetidae

P. chephrenis EP 0.001 0.003ns –

IP 0.21 1.61ns –

X. ramesis EP 0.06 0.41ns –

IP 0.06 0.36ns –

Muridae P. chephrenis EP 0.25 1.75ns –

IP 0.63 8.54* �0.006 ± 0.002

X. ramesis EP 0.75 15.01** �0.005 ± 0.001

IP 0.90 47.80** �0.01 ± 0.001





imago production in fleas feeding on an auxiliary host

was lower than in fleas feeding on the principal host,

except for auxiliary hosts that were most closely related

to the principal host (A. russatus for P. chephrenis and

G. andersoni for X. ramesis). Furthermore, egg and/or

new imago production in fleas exploiting an auxiliary

host decreased with an increase in phylogenetic distance

between the auxiliary and principal hosts. However, this

pattern was found only for auxiliary hosts confamilial

with the principal host, while reproductive performance

of fleas in an auxiliary host belonging to another family

(M. auratus) was surprisingly high.

Causes: within the principal host family

The principal host is a host species in which a parasite

achieves the highest abundance, that is, supports the

majority of individuals in a parasite population (Poulin

& Mouillot, 2004). The most likely reasons for this are

that a parasite is adapted to extract and use the

resources of this host and/or to overcome its antipara-

sitic defences. In addition, it has been demonstrated

that a parasite on a principal host can use resources

more efficiently than one on an auxiliary host and this

difference can affect fitness. Parapulex chephrenis spent

less energy for the digestion of blood of A. cahirinus

than of G. dasyurus (Sarfati et al., 2005) and produced

more eggs when fed on the former than on the latter

(Krasnov et al., 2002). However, the amount of energy

spent on resource processing does not appear to be

always associated with transforming the resource into

offspring. In this study, we found that flea fitness in

auxiliary hosts decreased with an increase in phyloge-

netic distance from the principal host (at least, within

host family), while fleas were found to spend less

energy for the digestion of blood of auxiliary hosts phy-

logenetically distant from the principal host than for

blood of close relatives (Khokhlova et al., 2012b). This

suggests that among-host variation in fitness of a hae-

matophagous ectoparasite can result from other than

energetic or nutritional reasons (e.g. Moloo et al., 1988;

but see Bize et al., 2008). For example, this variation

may be due to host differences in physiological and bio-

chemical properties of blood such as viscosity and pro-

tein, glucose or lipid contents. Indeed, some mosquitoes

prefer human over mouse blood because of differences

in the ratio of amino acids (Harrington et al., 2001).

Similarity in blood characteristics among closely related

hosts and concomitant difference in these characteris-

tics among distantly related hosts might be a mecha-

nism behind the pattern of flea fitness in different hosts

found in this study. We are not aware of any specific

study that compared blood composition among mam-

mals from a phylogenetic point of view. However,

higher levels of nucleic acids in birds and reptiles than

in mammals because of the nucleated red blood cells of

the former but not the latter (e.g. Lehane, 2005) sug-

gest that a correlation between blood properties and

phylogenetic distance among species may exist.

Another reason for differential fitness of a parasite in

different hosts may be among-host difference in anti-

parasitic defences. Ectoparasite fitness can be affected

by both behavioural (e.g. decreasing feeding success

and, consequently, ectoparasite fecundity due to inter-

rupting feeding bouts; Davies, 1990) and immunologi-

cal (e.g. Gouy de Bellocq et al., 2006) defences of a

host. If host defence abilities are phylogenetically con-

strained and, thus, are similar among closely related

species, then the reproductive output of a parasite

exploiting an auxiliary host would be higher if this host

would be closely related to than distantly related from

the principal host. This is because the parasite would be

subjected to a restricted set of host defences in the

former case, but a wider array of host defences in the

Phylogenetic distance from A. cahirinus

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Log

num

ber o

f offs

prin

g

(a)

0 20 40 60 80 100 120

40 50 60 70 80 90 100

Phylogenetic distance from M. crassus

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Log

num

ber o

f offs

prin

g

(b)

Fig. 3 Relationships between the mean number of offspring of a

female Parapulex chephrenis (a) and Xenopsylla ramesis (b) after

3 days of an uninterrupted stay on an auxiliary murine or

gerbilline host and phylogenetic distance between an auxiliary

host and the principal host (Acomys cahirinus for Parapulex

chephrenis and Meriones crassus for Xenopsylla ramesis). Phylogenetic

distances among hosts were calculated from branch length of the

phylogenetic tree of Bininda-Emonds et al. (2007). In this tree,

branch lengths are proportional to time.




latter case (Combes, 2001; Poulin & Mouillot, 2004).

However, the existence and strength of phylogenetic

signals (i.e. resemblance among phylogenetically related

species; Blomberg & Garland, 2002) in mammalian

antiparasitic defences remain to be studied.

Causes: outside the principal host family

Unexpectedly, egg and new imago production of both

fleas were high when they fed on a host heterofamilial

with the principal hosts (M. auratus). This could result

from the lack of defences against these fleas in this

host. Natural selection would not favour specific anti-

parasitic defences of a given host if it does not encoun-

ter this parasite or the frequency of parasite exposure is

low (Poulin et al., 1994; Shudo & Iwasa, 2001). Indeed,

results of experimental studies with rodent fleas forced

to feed on their preferred rodent host, a nonpreferred

rodent host or an ‘alien’ bat host, showed that fleas

performed better on a bat host than on the nonpre-

ferred rodent (Krasnov et al., 2007; Korine et al., 2012).

Furthermore, no cricetid host is naturally infested with

either P. chephrenis or X. ramesis. In addition, M. auratus

is a laboratory species, so it has not been attacked by

ectoparasites for many generations and, thus, its

defence tools could be weak. The latter explanation is,

however, weakened by the fact that some of rodents

used in this study were bred in our laboratory over

many generations, so their antiparasitic defences might

be affected similarly to M. auratus.

Evolutionary consequences: geographicaldistribution and host associations

Our study suggests that it may be advantageous for an

ectoparasite to select an auxiliary host which is either

the most closely or the most distantly (within some

realistic phylogenetic distance) related to the principal

host species. Evidence from distributional data of fleas

and their hosts provides support for both of such cases.

For example, among Palearctic fleas, the set of host spe-

cies used by a flea often appeared to be more taxonom-

ically clustered (i.e. the host species were more closely

related to each other) than in random subsets taken

from the regional pool of host species (Krasnov et al.,

2004b). Krasnov (2008) examined the change of principal

hosts across a geographical range in 177 flea species

from 49 geographical regions and evaluated between-

region change of the identity of the principal host spe-

cies and genus. It was found that the frequency distri-

bution of the mean number of the principal host

genera per region was much less left-skewed than the

mean number of the principal host species per region,

suggesting that the species that substitutes for the prin-

cipal host across the geographical range of a flea was

often phylogenetically (or at least taxonomically)

related (Krasnov, 2008). In other cases, a parasite

expanding its geographical range was found to switch

to a very distant relative of its principal host. For exam-

ple, Hoplopsyllus anomalus is a flea normally associated

with sciurid rodents (Spermophilus). However, in the

some areas of California, this flea is found mainly on a

heteromyid rodent (Dipodomys ingens; Tabor et al.,

1993).

Furthermore, the success of a parasite to invade a

new area has often been explained by ecological fitting

which occurs when parasites specialize on a resource

that is widespread among many host species, such as

blood, and track this resource rather than a host lineage

per se (Kethley & Johnston, 1975; Janzen, 1985; Brooks

et al., 2006a,b). Our results suggest that ecological fit-

ting may be phylogenetically restricted and depends on

phylogenetic relatedness of a potentially new host to an

original principal host. A parasite would colonize host

species that are either the most closely or the most dis-

tantly related to the principal host but not hosts with

intermediate phylogenetic relatedness to the latter.

A principal host: not only physiology

In both flea species, egg and/or new imago production

did not differ significantly between the principal host

and the most closely related auxiliary host (A. russatus

for P. chephrenis and G. andersoni for X. ramesis). More-

over, the new imago production in P. chephrenis was

significantly higher in the auxiliary host A. russatus

than in the principal host A. cahirinus. This suggests

that a principal host supports the majority of indivi-

duals in a parasite population not only due to physio-

logical reasons. For fleas and hosts in this study, these

additional reasons could be associated with habitat dis-

tribution and social behaviour of host species.

Although the reproductive performance of X. ramesis

on G. andersoni was as high as on M. crassus, this flea

does not co-occur naturally with the former host in

southern Israel (Krasnov et al., 1999). This is because

X. ramesis occupies mainly habitats with loess soils

(Krasnov et al., 1997), whereas G. andersoni is a strict

sand-dweller (Mendelssohn & Yom-Tov, 1999). Both

Acomys hosts of P. chephrenis occupy the same habitats.

The main ecological difference between these hosts is

that A. cahirinus nest communally, whereas A. russatus

is solitary (Mendelssohn & Yom-Tov, 1999; Shargal

et al., 2000). Host social behaviour may affect abun-

dance and distribution of contact-transmitted parasites

(Møller et al., 1993; Loehle, 1995; Ezenwa, 2004). In

particular, group leaving promotes contact between sus-

ceptible and infected individuals, so that increasing host

group size often results in increasing prevalence and/or

abundance of a parasite (Cote & Poulin, 1995). This

seems to be the case for P. chephrenis and A. cahirinus.

In conclusion, this study revealed proximate mecha-

nisms underlying the relationships between parasite

abundance in principal and auxiliary hosts and the




effect of phylogenetic positions of these hosts. Ecology,

geography and behavioural factors also play a role in

manifestation of these relationships.

Acknowledgments

This study was supported by the United States-Israel

Bi-National Science Foundation [grant number

2008142 to BRK, ISK and LJF]. The study was con-

ducted under permits from the Israel Nature and

National Parks Protection Authority (permits 2010/

37131 and 2011/38082) and the Ben-Gurion Univer-

sity Committee for the Ethical Care and Use of Ani-

mals in Experiments (authorization IL-52-07-2009).

This is publication no. 774 of the Mitrani Department

for Desert Ecology.

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Received 29 May 2012; accepted 25 June 2012




ectoparasite fitness in auxiliary hosts: phylogenetic distance from a principal host matters

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