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