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 Mullers 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 Male
XY Female XX Female heterogamety Female ZW Male
ZZ Distribution of sex chromosome heterogamety (numbers of families) Group XY males ZW females XY or ZW
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 Social Sex Determination female male 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 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 (TriversWillard 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 firstlaying 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)
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