karyotype differentiation and chromosomal variability in springtails (collembola, insecta)

Biol Fertil Soils (1990) 9 :119-125 Biology and Fertility

~ S o i l s �9 Springer-Verlag 1990

Karyotype differentiation and chromosomal variability in springtails (Coilembola, Insecta)* W. Hemmer **

Insti tut for Zoologie der Universit~it Wien, AlthanstrafJe 14, A-1090 Wien, Austria

Received July 23, 1989

Summary. Karyological studies in the mitotic comple- ments of arthropleonic springtails (Collembola) were car- ried out, applying chromosome-banding techniques (Giemsa C banding) for the first time. These techniques provide important indications on karyological variability and the genetic heterogeneity of local populations. Karyotype architecture may be of taxonomic significance at the species level. Chromosomal studies in various pop- ulations of the group Onychiurus armatus (Tullb.) high- light the importance of future karyological studies, espe- cially in taxonomically complicated groups.

New chromsome counts for 29 species of Collembola are presented. In 32 species karyotype morphology was analysed in detail. Acrocentric chromosomes were found to be an essential feature of karyotype evolution in Col- lembola. Generally, primitive taxa show a higher propor- tion of acrocentrics than more evolved species. For both Poduromorpha and Entomobryomorpha, ancestral karyotypes with only acrocentric chromosomes are pos- tulated. The most primitive karyotype found in the Poduromorpha (Ceratophysella bengtssoni, n = 7) con- sists of one metacentric and six acrocentric chromo- somes, probably derived from a hypothetical, purely acrocentric complement (n = 8) by a single centric fusion. In the Entomobryomorpha, acrocentric karyotypes (n = 7) are still found in Isotoma cinerea and in Pro- isotoma. Chromosome numbers exceeding n = 7 [Ony- chiuridae, n - - 7 - 9 ; Poduridae, n=(9 - )11 ] are sup- posed to be due to secondary increases.

Key words: Collembola - Chromosome numbers Karyotype evolution - Chromosome banding Chromosomal polymophisms

m

* Dedicated to the late Prof. Dr. W. Ktihnelt ** Present address: Institut for Botanik der Universitgt Wien, Rennweg 14, A-1030 Wien, Austria

Collembola are of great interest in ecology and soil biolo- gy because of their high abundance and rich eco- physiological diversity. Improved microsystematics, espe- cially of taxonomically complicated groups, seems to be indispensable for the purpose of differential characteriza- tion of edaphic ecosystems. In many animal groups systematics have been essentially improved by the study of chromosomal characters. Until now, karyological in- vestigations in springtails have been mainly concentrated on the study of polytene chromosomes in the Nea- nuridae. These studies have shown the significance and the usefulness of cytological parameters for the systematics of Collembola also (Cassagnau 1969; Dallai et al. 1983). The study of mitotic chromosomes as cytotaxonomic characters has, however, barely been touched so far (review by Fratello and Sabatini 1979). Few nuclei have been seriously karyotyped and there is no in- formation on chromosome banding.

The karyological approach to Collembola also seems to be worthwhile in the light of their specific phylogenetic importance. Detailed analyses of karyotype morphology and chromosome banding are expected to give informa- tion about phylogenetic pathways and the role of nuclear differentiation in the evolution of springtails.

Materials and methods

Specimens were collected in the field at various localities in north and east Austria (details in Hemmer 1987). The species determinations mainly followed Gisin (1960). The specimens were kept in small rearing jars with plaster bottoms at 1 2 - 2 0 ~ in the dark.

Most of the karyological data were obtained by the study of embry- onic tissue. Only chromosome counts in Onychiurus paradoxus and Onychiurus sibiricus rest on the analysis of meiosis. Chromosomes for banding studies and quantitative calculations were prepared by a modi- fied drop-technique: 1. Eggs were transferred into a microscopy basin containing a 0.5~ so- lution of tr i-sodium citrate dihydrate; the eggs were pierced or cracked by fine needles to allow infiltration, and left for 30 min in the dark at room temperature. 2. The hypotonized tissue was transferred into freshly prepared cold m e t h a n o l - 9 6 % acetic acid (3:1) by a pasteur pipette and fixed for 3 0 - 6 0 min. The fixative was changed at least once.

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3. For the chromosome preparation, small quantities of fixed tissue were quickly transferred to a clean microscope slide by a pasteur pipette. 4. During spreading of the fixative the tissue particles were softly crush- ed with a blunt needle or a minute spatula. Consequently, dividing cell aggregates were distributed over the slide by the spreading fixative. 5. To avoid too early drying and to guarantee good cell dispersion, small volumes of freshly prepared methanol-96~ acetic (4 : 1) were added at the same time. 6. When there were no more big clumps of cells the slide was gently blown dry by a medical rubber ball. Steps 3 - 6 were carried out under a stereoscopic microscope.

In addition, squash preparations in 4507o acetic acid or carmine ace- tic acid were made. Microscopic slides were either immediately analysed and/or photographed or made permanent by the dry ice method. Total staining of chromosomes was done in a 3-4070 solution of Giemsa's stain. The Giemsa C banding essentially followed Schwarzacher et al. (1980).

Photographs were taken on a Zeiss standard microscope. The mea- surement of chromosomes is based on high-magnification drawings of at least seven well spread metaphase plates. Relative chromosome lengths refer to the diploid female set (2n 9 = A A + X X = 100070). Cen- tromere indices were calculated after the formula c i = (length of long arm/total chromosome length)x 100, with the following terminology of centromere positions: ci_<62.5 metacentric; c i = 62.5-75.0 submeta- centric; c i = 75.0-87.5 subacrocentric; ci___87.5 acrocentric.

Results and Discuss ion

K a r y o t y p e e v o l u t i o n

T h e p r e s e n t k a r y o l o g i c a l s t u d y s u g g e s t s t h a t k a r y o t y p e e v o l u t i o n i n s p r i n g t a i l s c a n b e d e s c r i b e d a c c o r d i n g t o t h e o r i g i n a l s t a t u s o f t h e a c r o c e n t r i c c h r o m o s o m e t y p e w i t h - in t h i s t a x o n . T h e a n a l y s i s o f k a r y o t y p e m o r p h o l o g y in 32 spec ies s h o w e d a h i g h i n t e r s p e c i f i c v a r i a t i o n i n t h e

n u m b e r o f a c r o c e n t r i c s a n d i n d i c a t e d a s i g n i f i c a n t l y h i g h e r p r o p o r t i o n i n p r i m i t i v e t a x a (Tables 1 a n d 2).

C h r o m o s o m e c o m p l e m e n t s in p r i m i t i v e spec ies a re m a i n - ly o r exc lus ive ly c o m p o s e d o f a c r o c e n t r i c s , a n d t h e i r n u m b e r d e c r e a s e s in m o r e e v o l v e d t axa , b e i n g r e p l a c e d b y s u b a c r o - to m e t a c e n t r i c c h r o m o s o m e s . T h e p r e s e n t d a t a

a l l o w t h e p o s t u l a t i o n o f a n c e s t r a l k a r y o t y p e s w i t h o n l y

Table 2. Frequency of acrocentric chromosomes in Collembolan karyotypes, n, haploid chromosome number; a, number of acrocen- trics observed per haploid set. Karyological data on Neanuridae ac- cording to Cassagnau (1971, 1974b) and Dallai (1979)

?/ a

Table 1. New chromosome counts in Collembola (haploid numbers)

Poduridae Podura aquatica 11 a

Hypogastruridae Ceratophysella bengtssoni 7 Ceratophysella denticulata 7 Mesogastrura ojcoviensis 7 Xenylla grisea 7 Xenylla planipila 7

Onychiuridae Onychiurus paradoxus 7 Onychiurus furcifer 7 Onychiurus armatus 7 b Onychiurus fimatus 8 Onychiurus sibiricus 7 Onychiurus pseudogranulosus 7 Onychiurus scotarius 8

Isotomidae Isotoma cinerea 7 Isotoma notabilis 7 Proisotoma minuta 7 c Proisotoma subminuta 7 Isotomiella minor 6 Folsomia multiseta 7

Entomobryidae Sinella coeca 6 Willowsia buski 6 Entomobrya marginata 6 Entomobrya muscorum 6 Pseudosinella sp. 6 Lepidocyrtus paradoxus 6 Heteromurus nitidus 6 Orchesella cincta 6 Orchesella flavescens 6

Cyphoderidae Cyphoderus albinus 6

a n = 9 ( - 10), Brummer-Korvenkontio and Saure (1969) b n = 9, Sabatini and Fratello (1974) c n = 4, Nufiez (1962)

Hypogastruridae Hypogastrura viatica 7 5 Hypogastrura manubrialis 7 4 Ceratophysella bengtssoni 7 6 Ceratophysella denticulata 7 3 Mesogastrura ojcoviensis 7 6 Xenylla grisea 7 0 Xenylla planipila 7 0

Onychiuridae Onychiurus furcifer 7 1 Onychiurus armatus 7 1 - 3 Onychiurus fimatus 8 3 Onychiurus pseudogranulosus 7 1 Onychiurus scotarius 8 1

Neanuridae Neanura monticola 7 0 Bilobella grassei 6 1 Bilobella aurantiaca 7 0

Isotomidae Isotoma cinerea 7 7 lsotoma viridis 7 5 Proisotoma minuta 7 7 Proisotoma crassicauda 7 7 lsotomiella minor 6 5

Entomobryidae Sinella coeca 6 5 Willowsia buski 6 5 Pseudosinella sp. 6 5 Heteromurus nitidus 6 5 Lepidocyrtus paradoxus 6 4 Entomobrya nivalis 6 5 Entomobrya lanuginosa 6 4 Entomobrya marginata 6 2 Orchesella bifasciata 6 1 Orchesella cincta 6 1 Orchesella flavescens 6 0

Tomoceridae Tomocerus minor 6 5 Tomocerus flavescens 6 5

Cyphoderidae Cyphoderus albinus 6 2

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Fig. l a - k . Karyotype morphology and chromosome numbers in Poduromorpha ( a - e ) and En tomobryomorpha ( f -k ) : a Cerato- physella bengtssoni, o'2n = 13; primitive karyotype consisting of one large metacentric and six acrocentric chromosomes, centromeres of chromosomes I and II (_X) marked by arrows, NORs marked by arrow- heads; Giesma. b Ceratophysella denticulata, cy2n = 13; number of acrocentrics reduced to three pairs; Giemsa. c Xenylla grisea, ce2n = 13; all acrocentrics replaced by meta- to subacrocentric chromo- somes; Giemsa. d Podura aquatica, 2n = 22; Giemsa. e Bimodal karyotype of Tullbergia krausbaueri, 2n = 14; arrows indicate pairs of

nos. I and II; Giemsa. f Acrocentric complement of lsotoma cinerea, cy2n = 13; anaphase; phase contrast, g Sinella coeca, 9 2 n = 12; two metacentric chromosomes in the diploid set; anaphase; Giemsa. h Ento- mobrya lanuginosa, 9 2 n = 12; four metacentrics; anaphase; phase contrast, i Tomocerus minor, o~2n = 11; metacentric chromosome no. I marked by arrows; Giemsa. j Orchesellaflavescens, 9 2 n = 12; largely symmetrical karyotype without acrocentrics; Giemsa C banding, k Gi- emsa C-banded karyotype of Willowsia buski; 9 2 n = 1 2 ; the (sub)acrocentric no. I pair marked by arrows. Bars in c (for a - c and e - k ) and d: 10 g m

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Fig. 2a- i . Intraspecific chromosomal variability in Collembola: a Vari- able terminal Giemsa bands in chromosomes nos. I (X) and III (arrows) of Xenylla planipila, cy2n = 13. b lsotoma viridis, cy2n = 13; heterozygosity for a metacentric chromosome (arrow); the X chromo- some is represented by one of the three subacrocentrics, e Accessorial chromosomes in Isotomiella minor, ~2n = 11 + 6B; early metaphase; phase contrast, d - i Heterochromatin polymorphisms in three popula- tions of Onychiurus armatus (Tullb.), n = 7 (d "Wurzbachtal", e "Lainz", f - i "Klosterneuburg"); d heterozygosity for two interstitial

bands in chromosome III (arrows), cy2n = 13; e heterozygous terminal band in chromosome I (arrow), 92n = 14; f variable terminal band in chromosome III (arrows), not present in g - i , 92n = 14; g interstitial heterochromatin in the short arm of one chromosome I (arrow), 92n = 14; h heterozygous Giemsa bands in the long arm of chromo- some I and in chromosome VI (arrows), c~2n = 13; i heterozygosity for C bands in chromosomes I and II (arrows), 92n = 14. All plates but e are Giemsa C-banded. Chromosomes are numbered by arabic numerals in plates d - i . Bar (a-i): 10 Ixm

acrocen t r ic c h r o m o s o m e s for b o t h m a j o r phylogene t ic lines o f a r th rop leon ic Co l l embola , P o d u r o m o r p h a and E n t o m o b r y o m o r p h a . The ka ryo types o f several recent species can be easi ly u n d e r s t o o d f rom these or ig ina l sets.

The s implest c h r o m o s o m a l con f igu ra t ion in the P o d u r o m o r p h a was f o u n d in Ceratophysella bengtssoni (n = 7). Its ka ryo type is m a d e up o f one large metacen- tric, one long acrocentr ic , and five shor t acrocent r ic chro- m o s o m e s (Fig. 1 a). This cons te l l a t ion can be deduced f rom an acrocent r ic n = N.F. = 8 c o m p l e m e n t by a single centr ic fusion. Ka ryo type subdiv i s ion into two long and five shor t c h r o m o s o m e s is charac ter i s t ic also for Hypo- gastrura viatica, Hypogastrura manubrialis, and Ceratophysella denticulata (Fig. 1 b). Relat ive ch romo- some lengths are very s imi lar in all these species (Hem- mer 1987). Thus the decrease in the n u m b e r o f smal l acrocent r ics wi th in these t axa is l ikely due to per icen t r ic inversions. The lowest number o f acrocent r ic c h romo- somes was found in Ceratophysella denticulata, which is cons idered the mos t m o r p h o l o g i c a l l y special ized o f the

species s tudied (Bourgeois and Cassagnau 1972). In con- trast , the very high n u m b e r o f acrocentr ics wi th in Cerato- physella bengtssoni is consis tent wi th the view tha t it ho lds a basa l sys temat ic pos i t ion wi th in the genus. The lack o f acrocent r ic c h r o m o s o m e s within the genus (Xenylla grisea, Fig. l c; Xenylla planipila, Fig. 2a), which is the mos t advanced hypogas t ru r id t axon investi- ga ted (Cassagnau 1974a), cor responds to the present concep t ion o f ka ryo typ ic d i f ferent ia t ion. Fur the rmore , the p r o p o r t i o n o f acrocentr ics was genera l ly low in the Onych iur idae and also low in the N e a n u r i d a e (Table 2).

In the E n t o m o b r y o m o r p h a , qui te acrocent f ic comple - ments (n = 7) are still found in Proisotoma and l sotoma cinerea (Fig. I f). The relic charac te r o f these karyo types is evident by the fact tha t this c h r o m o s o m a l cons te l la t ion can be seen as the s tar t ing po in t o f ka ryo type evolut ion in the E n t o m o b r y i d a e and Tomocer idae (see below). In this way the ka ryo log ica l da t a essent ial ly coincide with current ideas on the phy logeny o f E n t o m o b r y o m o r p h a based on the s tudy o f chaetotaxy, suppos ing Isotoma,

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Proisotoma and Isotomina to be the most primitive taxa of the Isotomidae (Deharveng 1979; Szeptycki 1979).

The preponderance of acrocentric chromosomes in little-advanced species suggests the presence of a karyotypic orthoselection (White 1973) that favoured acrocentrics in the early evolution of springtails. The per- sistence of this trend may be seen in the frequent conser- vation of acrocentrics in the Isotomidae, Tomoceridae, and many Entomobryidae (Fig.1 f - i , k). Perhaps in some species metacentric chromosomes, which originated by chromosome fusion, have even reverted to (sub)acrocen- tric elements, re-establishing a +uniform acrocentric karyotype. This is assumed for Willowsia buski (Fig. 1 k) and Heteromurus nitMus (Hemmer 1987).

The high frequency of acrocentric chromosomes also highlights the cytological explanation for the trend to a reduction in chromosome numbers in the course of phy- logenetic differentiation in Collembola, which has been postulated by Nufiez (1962). Chromosome fusion is more likely to take place within acrocentric complements than chromosome fission, causing a general shift towards low- er numbers.

The establishment of type numbers

The small overall variation in chromosome numbers and the presence of characteristic type numbers in many fami- lies (Fratello and Sabatini 1979; Table 1) indicate that changes in chromosome numbers by Robertsonian rear- rangements took place only rarely in Collembola. These changes partly coincided with the main steps of anagenetic differentiation of Collembola, e.g., at the family level. In this way the evolution of the Tomoceridae and the Entomobryidae from primitive Isotomidae was accompanied by an alteration in the basal chromosome number. Centric fusion of two chromosomes in the an- cestral acrocentric complement (n = 7) to a modified karyotype (n = 6) having one metacentric and five acrocentric chromosomes was established in both phylo- genetic lines. This new chromosomal configuration, which may be taken as the basal karyotype of these taxa again, has been preserved in some genera of the En- tomobryidae (Sinella, Fig. 1 g; Pseudosinella) and per- haps in all species of Tomocerus (Fig. 1 i). The conspicu- ously differing relative length of the new chromosome I in the Entomobryinae (Sinella coeca 11.6%; Willowsia buski 10.7%; Pseudosinella sp. 10.2%; Heteromurus nitidus 11.1%) compared to Tomocerus (Tomocerus mi- nor 15.4~ Tomocerusflavescens 17.1%) strongly sug- gests that the reduction in chromosome number took place by Robertsonian fusion of different chromosomes in both groups.

The same type of karyotype transformation, i.e., the fusion of two acrocentrics in the hypothetical ancestral complement, led to the establishment of n = 7 as the type number for the Hypogastruridae. Considering the lack of chromosome number variation within this taxon, this could be taken as an important step in the evolution of the family. The largely uniform presence of n = 5 in sym- phypleonic springtails (Fratello and Sabatini 1979) may be a further instance of the phylogenetic bearing of

karyotypic differentiation at the level of chromosome number in Collembola.

Decrease and increase in chromosome numbers

According to the reduction concept of Nufiez (1962), rela- tively high chromosome numbers occurring in some Onychiuridae (n = 7, 8, 9; Frateilo and Sabatini 1979; Table 1) and in Podura aquatica (n = 11), Fig. 1 d; n = 9, according to Brummer-Korvenkontio and Saure 1969) were interpreted as primitive characters (Brummer-Kor- venkontio and Saure 1969; Sabatini and Fratello 1974). This seemed particularly clear for the monospecific Poduridae, which are usually regarded as an archaic relic family and were included in both the Poduromorpha and Symphypleona (Massoud 1976).

In the Onychiuridae, n = 7 is now shown to be the most frequent number and probably represents the origi- nal number of this family (Table 1), especially since it is the only number that is recorded for all genera of the family, Onychiurus s.l., Tullbergia s.l., and Tetrodon- tophora (Fratello and Sabatini 1979). All haploid num- bers higher than 7 in this group are hence more likely due to secondary increases. Moreover, the karyotypes of Tullbergia krausbaueri (Fig. 1 e) and Onychiurus armatus (Fig. 2d) resemble the chromosomal features of Hypogastrura and Ceratophysella. Karyotype subdivi- sion, separating two "macrochromosomes" and five "microchromosomes" in the hypogasturid genera (Fig. 1 a, b), can be observed in Tullbergia krausbaueri and in Onychiurus armatus, too, though the bimodality is less clearly expressed. Moreover, both in the hypogastrurid species and in Onychiurus armatus, chromosome II was found to represent the sex chromosome of the comple- ment ( 9 : 2 n = 12+XX; ce :2n = 12+XO; Figs. l a - b , 2d, e, and h), and chromosome I, which shows a clearly detectable secondary constriction in Hypogastrura and Ceratophysella, is probably a NOR-bearing chromosome in Onychiurus armatus as well. This indicates homology between these karyotypes and hints at a possible phyloge- netic junction between the Onychiuridae and the basal stock of the Hypogastruridae.

Referring to the present concept of acrocentric ances- tral complements throughout the Poduromorpha and En- tomobryomorpha ( n - - 7 - 8 ) , the presence of haploid number 11 in Podura is of great systematic interest. Per- haps the high chromosome number is the result of repeat- ed secondary increases, starting from n = 7 or n = 8, and hence could be taken as a karyotogicaI marker for the iso- lated phylogenetic differentiation of this family. Howev- er, the assumption of n = 11 as a primary character would mean that the Poduridae are a sister group of the Entomobryomorpha plus the rest of the "Poduromor- pha" (Uchida 1971), provided the establishment of acrocentric karyotypes near n = 8 in ancestral Hypo- gastruridae and Isotomidae has a monophyletic genesis. In any case, the strongly deviating chromosome number in Podura stresses its isolated systematic position. The presence of very low numbers (usually n = 5) in the Sym- phypleona makes the possible classification of the Poduridae into this taxon, as suggested by some authors

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(reviewed by Massoud 1976), less likely from the karyological point of view.

The concept of acrocentric karyotypes as the starting point of chromosomal differentiation in arthropleonic springtails harmonizes essentially with established ideas about the phylogeny of Collembola. It corresponds with the view that the hemi-edaphic living families Hypo- gastruridae and Isotomidae represent the ancestral stock of the two main developmental lines Poduromorpha and Entomobryomorpha, respectively (Cassagnau 1974a; Deharveng 1979), and morphologically primitive taxa can be identified by the higher proportion of acrocentric chromosomes. However, there is disagreement about the evaluation of some taxa by morphological and karyological features, as in the case of Podura or the Or- chesellinae (excluding Heteromurus), which are classified as primitive with regard to their cuticular characters (Szeptycki 1979) but obviously show highly derivative karyotypes, including a very low number of acrocentrics (Table 2, Fig. l j).

Taxonomic use of karyotype morphology

Apart from the reconstruction of phylogenetic pathways, chromosomal studies may also play an essential role in taxonomy in the future. Chromosome observations will supply differential diagnostic characters at the species level in systematic groups with some karyotypic diversity. Therefore cytotaxonomic studies may play an important role, especially in taxonomically complicated genera.

In studying several populations belonging to the com- plex of Onychiurus (Protaphorura) armatus, it became evident that a taxonomic classification based on chro- mosomal patterns does not necessarily coincide with clas- sification by traditional morphological characters (Hem- mer 1987). Hence the question arises of whether the pre- sent taxonomy is actually capable of defining species dif- ferences in this group by its morphological characters alone. Due to the presence of high intraspecific variability and the scarcity of taxonomically useful cuticular charac- ters, the congruence of morphologically and biologically defined species seems rather doubtful. Karyotype mor- phology may give useful indications of the biological compatibility of different specimens. Thus the intensified use of karyological parameters appears to be fundamen- tal in the systematics of Onychiuridae.

Intraspecific variation and chromosomal polymorphisms

Clear indications of intraspecific karyotypic heterogene- ity have been given by quantitative karyotype analyses and banding studies in five Austrian populations of Onychiurus (Protaphorura) armatus (Tullb.). Despite the small geographical spread of the populations investigated (all samples were taken in the immediate vicinity of Vien- na), significant differences in relative lengths and cen- tromere positions occur in some chromosomes (Table 3). In part these differences may be regarded as sufficient for a reduction in fertility of interbreeding populations. Ad- ditionally, the populations investigated differ to some ex- tent in their C-banding patterns.

Table 3. Morphometric karyotype variation in five populations of Onychiurus armatus (Tullb.), n 7, from the vicinity of Vienna

Species/ Chromosome Relative Centromere locality no. length ( % / 2 n ) index

O. armatus I 12.62 _+ 0.48 63.2 _+ 1.9 "Wurzbachtal" III 7.01 _+0.52 74.1 _+2.7

IV 6.73 _+ 0.45 Acrocentric VII 4.64 + 0.41 Acrocentric

O. armatus I 12.69 _+ 0.75 62.5 -+_ 2.5 "Lainz" III 6.83 _+ 0.37 73.9 + 3.5

IV 6.69 +_ 0.50 Acrocentric VII 4.60 _+ 0.32 Acrocentric

O. armatus I 14.46_+ 1.13 71.1 _+2.6 "Lobau 1" III 6.32 _+ 0,38 75.2 _+ 3.7

IV 5.67_+0.54 81.2+3.8 VII 4.39 _+ 0.41 Acrocentric

O. armatus I 12.91 _+0.41 61.6_+ 1.7 "Lobau 2" III 7.18 -+ 0.25 74.9 +_ 3.3

IV 6.98 _+ 0.24 Acrocentric VII 4.47 +_ 0.38 Acrocentric

O. armatus I 10.50 -+ 0.68 63.8 -+ 1.9 "Klosterneuburg" III 6.69 -+ 0.42 61.6 -+ 2.0

IV 7.63 _+ 0.33 Acrocentric VII 5.41 _+ 0.45 Acrocentric

The data are consistent with the view that low mobili- ty of animals and high seasonal fluctuations in popula- tion size, favouring kin group founding, may promote the fixation of chromosomal rearrangements and thus lead to stepwise isolation of local populations (White 1973, 1978; Hedrick and Levin 1984).

The existence of manifold karyotypic variability with- in single populations is probably of great importance in the understanding of Collembolan population genetics. Apart from chromosomal dimorphism (Fig. 2b) and the occurrence ~f B-chromosomes (Fig. 2c) this variability mainly comprises heterochromatin polymorphisms (Fig. 2a, d - i ) . Though heterochromatic segments (including B-chromosomes, which are usually composed of hetero- chromatin only), do not themselves have genetic func- tions, this DNA may be of great importance for popula- tion genetics by its various nucleotypic effects (Loidl 1987). Many of the chromosomal variants detected obvi- ously never occur in a homozygous constellation but only in genotypes showing chromosomal heterozygosity. The wide systematic dispersion of the observed polymor- phisms suggests that karyotypic variability possibly acts as an important genetic regulatory mechanism in springtails.

Acknowledgments . I am grateful to Dr. J. Loidl (Vienna) and Dr. R. Samuel (Vienna) for critically reading the manuscript. This work was supported by the Austrian Fonds zur F6rderung der wissenschaftlichen Forschung (P6237B).

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