Sex and Sex Ratio

• What is sex?

• Why sex?

• Mechanisms of sex (gender) determination

• Sex (gender) ratio allocation

What is sex?Genetic recombination

What determines gender?

Why Sex? The Costs.

1. Cost of males - could produce twice as many females

2. Cost of meiosis - recombination breaks up favorable gene complexes

3. Finding a mate costs time and energy

Why Sex? The Benefits.

1. Prevents Muller’s Ratchet– deleterious mutations accumulate in

asexual lineages, not in sexuals– Provides long term benefit

2. Novel gene combinations are created– Red Queen Hypothesis: need to create

new gene arrangements to combat pathogen evolution

– Provides short term benefit

Host sexuality and parasitism

• Some spp of snails are both sexual and asexual• Sexual proportion (freq. of males) increases with

infection by trematode parasites (Lively 1992)

Freq. of males=sexuals Trematode infections

Sex determination

• Types– Genetic (either chromosomal or genic)– Environmental– Social

• Influences the degree to which a female can alter the sex ratio of her offspring

Chromosomal sex determination

Female Male

ZW ZZ

Female heterogamety

Male Female

XY XX

Male heterogamety

Distribution of sex chromosome heterogamety

(numbers of families)

Group XY males ZW females XY or ZWBirds all

Butterflies all

Mammals all

Flies 7 3

Fish 19 10 2

Lizards 4 3 1

Amphibians 2 3 3

Bull, 1983; Solari 1994

Haplodiploidy• Mechanism

– Haploid males develop from unfertilized eggs– Diploid females develop from fertilized eggs

• Distribution– all hymenoptera, thrips, scale insects, some beetles

Creates asymmetriesin relatedness

Environmental Sex Determination

Incubation

temperature– Terrapins

– Turtles

– Alligators

Social Sex Determination

• Many fish can undergo sex reversal

• Depends on mating system

• Example: dominant female in Anthias sea bass harem changes into male when territorial male disappears

female

male

Sex Ratio Allocation: Null Model

Produce more males Produce more females

Sex Ratio Allocation: Null Model

• R. A. Fisher: Parental strategies should evolve towards equal investment in offspring of the two sexes

• If sex ratio falls below 50%, increased production of rare sex is favored

• Assuming random mating, rare sex will experience greater reproductive success

• Frequency-dependent selection leads to a 1:1 stable sex ratio

Adaptive Sex Ratio Bias

• Maternal condition influences offspring investment (Trivers-Willard Effect)

• Local mate competition

• Local resource competition

• Local resource enhancement

Trivers-Willard Effect

• Population sex ratio is 1:1, but individual sex allocation depends on condition

• If moms in good condition transfer competitive ability to sons more than daughters (e.g. through parental care)

• and dominant individuals sire more offspring• then, they should produce more sons than daughters• and females in poor condition should produce more

daughters

Condition-dependent sex allocation in red deer

Dominance dependent sex allocation in yellow baboons

Dominant females have more daughters than sons

(pass social rank to daughters)

Local Mate Competition• Mating b/w siblings takes

place near hatching site • Brothers compete with each

other for mating• Solution: Produce few sons.

– Expect most offspring of first-laying female to be daughters

– Expect second-laying female to adjust sex ratio according to the proportion of brood that are hers

• Fig wasps, parasitoid waspshttp://waynesword.palomar.edu/pljune99.htm

Nasonia wasps adjust sex ratio

Line gives theoretical predictionassuming females contributeequal numbers of offspring and adjust sex ratio to maximize inclusive fitness.

Second laying females adjust sex ratio proportional to offspring produced

Sex ratio depends on proportion of eggs that belong to the second female

Local Resource Competition

• Offspring that stay near their birth site may compete with their parents for resources

• In many species, one sex disperses farther or at a greater rate than the other sex

• Solution: Produce more of the dispersing sex

• For example, galagos (bush babies) produce more males

Local Resource Enhancement

• In some spp, offspring of one sex delay dispersal and remain at the natal site to help parents raise their siblings

• Benefits of helpers must be greater than cost of increased competition

• Ex: Red-cockaded woodpecker groups are male-biased– Males help feed young– Available nesting cavities are rare

Seychelles warbler sex ratios

As territories fill up, females on good territories produce fewer males (which disperse) but more females (which help)