Sex Ratio Evolution – Deviations From 1-1

Sex ratio evolution – deviations from 1:1 Parasitoid wasps – …females lay eggs inside a single host (e.g., caterpillar or fig fruit) and the siblings hatch, develop, and mate before they emerge from the host …new young females then seek out new hosts Such species often have extraordinarily female-biased sex ratios Sex ratio evolution – deviations from 1:1 William Hamilton (1967) explained these biased sex ratios by a phenomenon he called local mate competition. Typically, in a large population, the sons of a female compete for mates with the sons of many other females. However, under local mate competition, sons compete only with each other in a small group founded by their mother. Females can increase fitness by producing mostly daughters, with only enough sons to inseminate them assume that each female produces 100 eggs Fitness = 95 * 100 = 9500 Fitness = 50 F * 100 = 5000 95 F 5M Fitness = 5 * 100 = 500 50 F 50 M 5F 95 M Sex ratio of offspring Sex ratio evolution – deviations from 1:1 • helpers remain in parental nest & assist with foraging, incubation, etc. • 88% of all helpers are female; males typically disperse to new habitats local resource enhancement - when one sex (of offspring) improves their parents reproductive success Seychelles warbler - Komdeur (1996) demonstrated local resource enhancement & the context-dependent effect of helpers Acrocephalus secheliensis – Seychelles warbler Endemic to 4 islands in Seychelles islands (~60 km N Madagascar) high-quality territories (many insects) low-quality territories (few insects) 1-2 nest helpers INCREASE survival & the reproductive success of a nest helpers DECREASE the reproductive success of the nest when helpers are beneficial (high-resource), parents produce more of the helping sex when helpers are not beneficial (low-resource), parents produce more of the non-helping sex territory quality # daughters # sons % sons LOW 13 44 77% HIGH 32 4 12.5% insectivorous n=118 nests (observed over 3 years) sex-ratio varies w/ territory quality Relocation experiment… manipulation: •! move birds from low-quality territory to a high-quality territory (experimental) Gerrid waterstriders engage in violent pre-mating struggles, which are costly… – increased predation •! move birds from high-quality territory to another high-quality territory (control) – direct injury, energy loss prediction: – decrease female’s ability to choose a mate •! experimental birds will adjust sex ratio & begin producing MORE females experimental group (low!high) produced more females following relocation control group (high!high) did not change after relocation •! Males have evolved elaborate ‘armaments’ (spikes, hooks, increased size) Before relocation females males LOW 2 18 HIGH 15 4 After relocation females males HIGH 29 5 HIGH 16 4 •! Females have evolved countermeasures (spines, increased size, etc.) When helpers are beneficial (high-resource environments), natural selection favors parents who produce more of the helping sex (females). Start with familiar assumption that females invest substantially more in offspring production compared to males; thus, females should be more selective in mate selection. Arnqvist & Rowe (2002) Measured armament size and body size in 15 species Even though we know that females can benefit from multiple matings (in some species), there are costs to mating multiple times as well. Some of the costs include… – predicted when there was an imbalance, it would predict the outcome of mating struggles If multiple prolonged matings reduce the overall fitness of females MORE SO than in males, then resistance to mating by females should evolve under certain circumstances. In contrast, males could increase fitness by evolving measures to overcome resistance. Leads to potential conflict… •! When females had bigger defenses relative to male ‘weapons’ there was little difference between treatments Excess males Equal sex ratio Compared the difference in mating outcomes between the two treatments to determine which sex was in control •! When females were outsized by males they observed more costly struggles & more mating in the excess male treatment Results suggest sexual conflict over control of mating & the conditions for antagonistic coevolution… (1)!As the cost to females of mating increases, females evolve behavioral resistance to male mating attempts (2)!In response, males evolve measures to overcome female resistance (3)!An “arms race” ensues, and females evolve morphological counter-adaptations to the male measures Proposed sequence of events… Dytiscidae, giant diving beetle Males in some species pounce upon any females encountered. Females counter by rapid and erratic swimming, including attempts to dislodge the male by driving him into the substrate. In other species, males have ‘suction cups’ to grip females (during mating, or use them to mateguard females following copulation). M Females & males sexually dimorphic for surface of elytra – countermeasure to male behavior ? M F M M F F F All females (BOTH treatments) alive @ beginning of experiment LOW mating treatment FIVE independent evolutionary origins of sexually dimorphic wing coverings Fruit fly, Drosophila melanogaster HIGH mating treatment 2 treatments • varied mating frequency Rates of egg production & egg fertility were EQUAL for females in the different treatments. sucker-shaped male proand mesotarsal setae The only difference was the amount of re-mating females did. TWO reversals back to sexually monomorphic wing coverings HIGH mating (1 F & 2 M’s, 4 days) LOW mating 1 F + 2 M’s, 1 day 1 F + 2 emasc M’s, 3 days New males were substituted every four days (both treatments) Toxic seminal fluid in males… Toxic seminal fluid in males… • Increases the egg laying rate of females • Reduces the tendency of females to re-mate • Increases the egg laying rate of females • Reduces the tendency of females to re-mate Countermeasures in females… Countermeasures in females… • Females become resistant to toxic sperm • Females become resistant to toxic sperm Experimental (selected) lines Control (unselected) lines Males selected for toxic seminal fluid Males NOT under selection for toxicity UNEQUAL sex ratio (male biased) EQUAL sex ratio (1:1) ca. 30 generations ca. 30 generations only MALE offspring kept across generations only FEMALE offspring kept across generations (males selected for high toxicity) (females NOT selected for resistance) selected males have higher fitness than unselected males females had higher mortality when mated to selected males (those w/ toxic seminal fluid) compared to unselected males