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Sex and Sex Ratio

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. Cost of males - could produce twice as many females

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Sex and Sex Ratio

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

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

  3. Why Sex? The Costs. • Cost of males - could produce twice as many females • Cost of meiosis - recombination breaks up favorable gene complexes • Finding a mate costs time and energy

  4. Why Sex? The Benefits. • Prevents Muller’s Ratchet • deleterious mutations accumulate in asexual lineages, not in sexuals • Provides long term benefit • Novel gene combinations are created • Red Queen Hypothesis: need to create new gene arrangements to combat pathogen evolution • Provides short term benefit

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

  6. 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

  7. Female heterogamety Female Male ZW ZZ Chromosomal sex determination Male heterogamety Male Female XY XX

  8. Distribution of sex chromosome heterogamety(numbers of families) Bull, 1983; Solari 1994

  9. Creates asymmetries in relatedness Haplodiploidy • Mechanism • Haploid males develop from unfertilized eggs • Diploid females develop from fertilized eggs • Distribution • all hymenoptera, thrips, scale insects, some beetles

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

  11. 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

  12. Sex Ratio Allocation: Null Model Produce more males Produce more females

  13. 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

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

  15. 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

  16. Condition-dependent sex allocation in red deer

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

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

  19. Nasonia wasps adjust sex ratio Line gives theoretical prediction assuming females contribute equal numbers of offspring and adjust sex ratio to maximize inclusive fitness.

  20. Second laying females adjust sex ratio proportional to offspring produced Sex ratio depends on proportion of eggs that belong to the second female

  21. 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

  22. 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

  23. Seychelles warbler sex ratios As territories fill up, females on good territories produce fewer males (which disperse) but more females (which help)

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