Sex and Sex Ratio

1. Sex and Sex Ratio • What is sex? • Why sex? • Mechanisms of sex determination • Sex ratio allocation

2. What is sex? Genetic recombination What determines gender?

3. Why Sex? The Costs. • Cost of males - could produce 2 times as many females • Cost of meiosis - favorable gene complexes recombine • Energetic and time costs of finding a mate

4. Why Sex? The Benefits. • Muller’s Ratchet • deleterious mutations accumulate in asexual lineages • Genetic Recombination • novel gene combinations are created • Red Queen Hypothesis: need to create new gene arrangements to combat pathogen evolution

5. Evolutionary Arms Race • Some spp of snails are both sexual and asexual • Sexual proportion (freq. of males) increases with parasite infection

6. Sex determination • Influences the degree to which a female can alter the sex ratio of her offspring • Types • Chromosomal • Environmental • Social

7. Chromosomal Sex Determination • Heterogamety • Male: (XY) mammals, flies • Female: (ZW) Birds, Butterflies

8. Chromosomal Sex Determination • Haplodiploidy: all hymenoptera, thrips, scale insects, some beetles • Haploid males develop from unfertilized eggs • Diploid females develop from fertilized eggs Creates asymmetries in relatedness

9. Environmental Sex Determination • Incubation temperature • Terrapins • Turtles • Alligators

10. Social Sex Determination • Some fish undergo sex reversal • Depends on mating system • Ex: dominant female in sea bass harem changes into male when territorial male disappears

11. 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 an ESS of 1:1 sex ratio

12. Adaptive Sex Ratio Bias • Maternal condition influences offspring investment (Trivers-Willard Effect) • Local mate competition • Local resource competition • Local resource enhancement

13. 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 • Then, they should produce more sons than daughters assuming that • Dominant individuals sire more offspring

14. Condition-dependent sex allocation in red deer

15. Dominance dependent sex allocation in yellow baboons • Dominant females have more daughters than sons (pass social rank to daughters)

16. Local Mate Competition • Mating b/w siblings takes place near hatching site • Males compete with each other for matings. • Solution: Produce few sons. Expect most offspring of first-laying female to be daughters • What if a second female arrives to lay eggs? • Ex: Nasonia wasps; haplodiploid ‘super-parasites’ on fly pupae

17. Depends on proportion of eggs that belong to the second female

18. 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 (Ex: galagos produce more males)

19. 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 male-biased • Prediction: Sex-ratio biased towards helping sex on high-quality territories?

20. Seychelles Warblers As territories fill up, produce fewer males (which disperse) but more females (which help)