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 Freq. of males=sexuals Trematode infections • Some spp of snails are both sexual and asexual • Sexual proportion (freq. of males) increases with infection by trematode parasites (Lively 1992)

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

Male heterogamety Female heterogamety

Male Female Female Male XY XX ZW ZZ

Group Birds Butterflies Mammals Flies Fish Lizards Amphibians Distribution of sex chromosome heterogamety (numbers of families) XY males ZW females XY or ZW all all all 7 19 4 2 10 3 3 3 2 1 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 asymmetries in relatedness

Environmental Sex Determination Incubation temperature – Terrapins – Turtles – Alligators

female

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 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 wasps http://waynesword.palomar.edu/pljune99.htm

Nasonia

wasps adjust sex ratio Line gives theoretical prediction assuming females contribute equal 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)