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CHAPTER 8: SEX AND EVOLUTION. Stalk-eyed flies. Stalk-eyed flies. Both M &F have these stalks In some species – they are up to twice as long in males as they are in females (see pic ->) Why does this difference between the sexes exist?. Background.
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Stalk-eyed flies • Both M &F have these stalks • In some species – they are up to twice as long in males as they are in females (see pic ->) • Why does this difference between the sexes exist?
Background • Among the most fascinating attributes of organisms are those related to sexual function, such as: • gender differences • sex ratios • physical characteristics and behaviors that ensure the success of an individual’s gametes
Sexual reproduction mixes genetic material of individuals. • In most plants and animals reproduction is accomplished by production of male and female haploidgametes (sperm and eggs): • gametes are formed in the gonads by meiosis • Gametes join in the act of fertilization to produce a diploidzygote, which develops into a new individual.
Asexual Reproduction Progeny produced by asexual reproduction are usually identical to one another and to their single parent: This fern sprouts a fully formed plant from the tip of a leaf • asexual reproduction is common in plants (individuals so produced are clones) • many simple animals (hydras, corals, etc.) can produce asexual buds, which: • may remain attached to form a colony • may separate to form new individuals
Other Variants on Reproduction • Asexual reproduction: • production of diploid eggs (genetically identical) without meiosis (common in fishes, lizards and some insects) • production of diploid eggs (genetically different) by meiosis, with suppression of second meiotic division • self-fertilization through fusion of female gametes • Sexual reproduction: • self-fertilization through fusion of male and female gametes (common in plants)
Sexual reproduction is costly. • Asexual reproduction is: • common in plants • found in all groups of animals, except birds and mammals • Sexual reproduction is costly: • gonads are expensive organs to produce and maintain • mating is risky and costly, often involving elaborate structures and behaviors
Sexual reproduction is costly. So why does sexual reproduction exist at all?
Cost of Meiosis 1 • Sex has a hidden cost for organisms in which sexes are separate: • only half of the genetic material in each offspring comes from each parent • each sexually reproduced offspring contributes only 50% as much to the fitness of either parent, compared to asexually produced offspring • this 50% fitness reduction is called the cost of meiosis • for females, asexually produced offspring carry twice as many copies of her genes as sexually produced offspring: • thus, mating is undesirable
Cost of Meiosis 2 • The cost of meiosis does not apply: • when individuals have both male and female function (are hermaphroditic) • when males contribute (through parental care) as much as females to the number of offspring produced: • if male parental investment doubles the number of offspring a female can produce, this offsets the cost of meiosis
Advantages of Sex • One advantage to sexual reproduction is the production of genetically varied offspring: • this may be advantageous when environments also vary in time and space • Is this advantage sufficient to offset the cost of meiosis?
Who’s asexual? If asexual reproduction is advantageous, then it should be common and widely distributed among many lineages: • most asexual species (e.g., some fish, such as Poeciliopsis) belong to genera that are sexual • asexual species do not have a long evolutionary history: • suggests that long-term evolutionary potential of asexual reproduction is low: • because of reduced genetic variability, asexual lines simply die out over time
Why have sex? • By the late 1980s, in the contest to explain sex, only two hypotheses remained in contention. • One… the deleterious mutation hypothesis • sex exists to purge a species of damaging genetic mutations; Alexey Kondrashov (at the National Center for Biotechnology Information) argues that in an asexual population, every time a creature dies because of a mutation, that mutation dies with it. In a sexual population, some of the creatures born have lots of mutations and some have few. If the ones with lots of mutations die, then sex purges the species of mutations. Since most mutations are harmful, this gives sex a great advantage. • But… But why eliminate mutations in this way, rather than correcting more of them by better proofreading? • Kondrashov: It may be cheaper to allow some mistakes through and remove them later. The cost of perfecting proofreading mechanisms escalates as you near perfection.
But… • According to Kondrashov's calculations, the rate of deleterious mutations must exceed one per individual per generation if sex is to earn its keep eliminating them; if less than one, then his idea is in trouble. • The evidence so far is that the deleterious mutation rate teeters on the edge: it is about one per individual per generation in most creatures. • But even if the rate is high enough, all that proves is that sex can perhaps play a role in purging mutations. It does not explain why sex persists. • The main defect in Kondrashov's hypothesis is that it works too slowly. Pitted against a clone of asexual individuals, a sexual population must inevitably be driven extinct by the clone's greater productivity, unless the clone's genetic drawbacks can appear in time. Currently, a great deal of effort is going into the testing of this model by measuring the deleterious mutation rate, in a range of organisms from yeast to mouse. But the answer is still not entirely clear.
Sex and Pathogens • The evolution of virulence by parasites that cause disease (pathogens) is rapid: • populations of pathogens are large • their generation times are short • The possibility exists that rapid evolution of virulence by pathogens could drive a host species to extinction.
The Red Queen Hypothesis • Genetic variation represents an opportunity for hosts to produce offspring to which pathogens are not adapted. • Sex and genetic recombination provide a moving target for the evolution by pathogens of virulence. • Hosts continually change to stay one step ahead of their pathogens, likened to the Red Queen of Lewis Carroll’s Through the Looking Glass and What Alice Found There. • ‘it takes all the running you can do, to keep in the same place.’
Sex vs Asex • One of the main proponents of the Red Queen hypothesis was the late W. D. Hamilton. • In the late 1970s, with the help of two colleagues from the University of Michigan, Hamilton built a computer model of sex and disease, a slice of artificial life. It began with an imaginary population of 200 creatures, some sexual and some asexual. Death was random. Who won? • As expected, the sexual race quickly died out. In a game between sex and "asex," asex always wins -- other things being equal. That's because asexual reproduction is easier, and it's guaranteed to pass genes on to one's offspring.
Now add parasites • Next they introduced 200 species of parasites, whose power depended on "virulence genes" matched by "resistance genes" in the hosts. • The least resistant hosts and the least virulent parasites were killed in each generation. • Now the asexual population no longer had an automatic advantage -- sex often won the game. It won most often if there were lots of genes that determined resistance and virulence in each creature. • In the model, as resistance genes that worked would become more common, then so too would the virulence genes. Then those resistance genes would grow rare again, followed by the virulence genes. As Hamilton put it, "antiparasite adaptations are in constant obsolescence." But in contrast to asexual species, the sexual species retain unfavored genes for future use."The essence of sex in our theory," wrote Hamilton, "is that it stores genes that are currently bad but have promise for reuse. It continually tries them in combination, waiting for the time when the focus of disadvantage has moved elsewhere."
Real-world evidence • asexuality is more common in species that are little troubled by disease: boom-and-bust microscopic creatures, arctic or high-altitude plants and insects. • The best test of the Red Queen hypothesis, though, was a study of a little fish in Mexico called the topminnow. The topminnow, which sometimes crossbreeds with another similar fish to produce an asexual hybrid, is under constant attack by a worm that causes "black-spot disease." The asexually reproducing topminnows harbored many more black-spot worms than did those producing sexually. • That fit the Red Queen hypothesis: The sexual topminnows could devise new defenses faster by recombination than the asexually producing ones.
Parasites and sex in freshwater snails • One of the most compelling tests in the Red Queen Hypothesis has been conducted by Curt Lively and his coworkers at Indiana University • Test focuses on the freshwater snail (P. antipodarum) • Most are asexual, all-female clones • Populations in some localities have ~ 13% males – enough to maintain some genetic diversity • Trematode worms of the genus Microphallus infect the snails and sterilize them • Hosts in the life cycle of the worm are ducks
Snails and parasites • Asexual snails reproduce faster than sexual individuals • Where the prevalence of Micorphallus infection is high sexual individuals are common • Why? asexual clones cannot persist in the face of high rates of parasitism • Why?
experiment • ? if the parasites had evolved to specialize on local (depth-specific) populations of snails, then they should have the greatest success in infecting the populations they evolved with • Took snails from 3 different depths – exposed them to parasites obtained from each group of snails • Remember: the definitive hosts (ducks) feed mostly in shallow water, and so only the shallow-water parasite populations cycled regularly through snail host populations
More on sex and evolution • a 2005 study shows that sex leads to faster evolution. • To demonstrate this, a team of scientists created a mutant strain of yeast that, unlike normal yeast, was unable to divide into the sexual spores that allow yeast to engage in sexual reproduction. Yeast can reproduce either sexually or asexually. • When testing this mutant strain in stress-free conditions, the scientists found that it performed as well as normal yeast. In more extreme conditions, however, the normal yeast grew faster than the asexual mutants. • This shows "unequivocally that sex allows for more rapid evolution," said Matthew Goddard of the School of Biological Sciences at the University of Auckland in New Zealand.
Perhaps… • It could well be that the deleterious mutation hypothesis and the Red Queen hypothesis are both true, and that sex serves both functions. • Or that the deleterious mutation hypothesis may be true for long-lived things like mammals and trees, but not for short-lived things like insects, in which case there might well be need for both models to explain the whole pattern. • Perpetually transient, life is a treadmill, not a ladder.
Individuals may have female function, male function, or both. • The common model of two sexes, male and female, in separate individuals, has many exceptions: • hermaphrodites have both sexual functions in the same individual: • these functions may be simultaneous (plants, many snails and most worms) or • sequential (mollusks, echinoderms, plants, fishes)
Sexual Functions in Plants • Plants with separate sexual functions in separate individuals are dioecious: • this condition is relatively uncommon in plants • Most plants have both sexual functions in the same individual (hermaphroditism): • monoecious plants have separate male and female flowers • plants with both sexual functions in the same flower are perfect (72% of plant species) • most populations of hermaphrodites are fully outcrossing (fertilization takes place between gametes of different individuals) • Many other possibilities exist in the plant world!
Separate Sexes versus Hermaphroditism • When does adding a second sexual function (becoming hermaphroditic) make sense? • gains from adding a second sexual function must not bring about even greater losses in the original sexual function • this seems to be the case in plants, where basic floral structures are in place • for many animals, adding a second sexual function entails a net loss in overall sexual function
Check it out: more on the web Sequential hermaphroditism Some organisms are male first and then become female later in their lives Some organisms are female first and then become male later in their lives
From ‘more on the web’ In most organisms, including humans, sex is determined by genetic factors. Nature is endlessly inventive, however, and in some species of reptiles and fish the temperature at which the egg develops determines sex. In the case of reptiles, females lay their eggs in sand or dirt and the temperature of the soil at the depth at which the clutch is laid determines the temperature of the eggs. The mechanism of temperature-dependent sex determination (TSD) has not been worked out fully, but it involves an effect of temperature on gene expression in the developing gonads of the embryo. Each species with TSD has a critical temperature, usually around 28°-30°C, below which offspring become one sex and above which offspring become the other sex. Whether high temperatures produce males or females depends on the species, although in most cases higher temperatures produce females. Why do you think that some groups of animals would adopt TSD? How is temperature likely to affect the length of the embryo growth period and the relative size of the hatchling, and how would these factors differentially affect the fitness of male and female offspring? Would it be possible for an egg-laying female to control the sex of her offspring? What factors might constrain the temperature regime that a female can provide for her clutch of eggs? How would a string of unusually hot or cold seasons affect sex ratio in a population, and how should females respond to this in determining where to lay their eggs? A recent discussion of these issues can be found in R. Shine (1999).
Whether a sequential hermaphrodite is first male or first female depends on how reproductive success through male or female function potentially increases with increasing body size. Larger females have larger reproductive organs and can lay more eggs. Small males can have high reproductive success where fertilization is internal and males do not contest social status, as in the case of the slipper shell. Where males compete for territories, as in the wrasse, large size is prerequisite to successful reproduction.
Sequential hermaphroditism reflects changes in the costs and benefits of male and female sexual function as an organism grows. In some marine gastropods having internal fertilization, such as the slipper shell Crepidula, insemination requires the production of only small amounts of sperm. Hence male function consumes few resources and has little effect on growth. Consequently, individuals of many such species are male when they are small and become female when they are large and thus able to produce correspondingly large clutches of eggs For others…
Mechanisms of sex determination • Inheritance of sex-specific chromosomes (eg: humans, birds…) • Other factors… • Competition among X- and Y-bearing sperm to fertilize eggs or selection abortion of male or female embryos • Birds: control is so precise that the chance of an offspring being male changes predictably from the first to the last laid egg in the clutch – as a way of controlling (maybe) competitive interactions between male and female siblings
Mechanisms of sex determination • Determined by the physical environment • Several species of turtles, lizards, alligators: sex determined by the temperature at which it develops in the egg • Embryos that develop at lower temp produce males; higher temp: females; for turtles. Opposite for alligators and lizards. • Hmm… ? • Determined by the social environment • The wrasse (discussed earlier) is a sequential hermaphrodite • Raised in isolation females; raised in small groups, at least one develops initially into a male w/o passing through a female phase • Females may become males later when they grow large enough to compete for territories ; primary males never change their sex
Sex ratio of offspring is modified by evolution. • When sexes are separate, sex ratio may be defined for progeny of an individual or for the population as a whole. • Sex ratio: number of males relative to the number of females • Humans have 1:1 male:female sex ratios, but there are many deviations from this in the natural world. • Despite deviations, 1:1 sex ratios are common. Why? • Every product of sexual reproduction has one father and one mother • if the sex ratio is not 1:1, individuals belonging to the rarer sex will experience greater reproductive success: • such individuals compete for matings with fewer individuals of the same sex • such individuals, on average, have greater fitness (contribute to more offspring) than individuals of the other sex
1:1 Sex Ratios: An Explanation Consider a population with an unequal sex ratio... • individuals of the rare sex have greater fitness • mutations that result in production of more offspring of the rare sex will increase in the population • when sex ratio approaches 1:1, selective advantage of producing more offspring of one sex or another disappears, stabilizing the sex ratio at 1:1 • this process is under the control of frequency-dependent selection
Why do sex ratios deviate from 1:1? • One scenario involves inbreeding: • inbreeding may occur when individuals do not disperse far from their place of birth • a high proportion of sib matings leads to local mate competition among males
Female condition and offspring • In some situations, a parent may benefit from a skewed sex ratio among its progeny, meaning that it should produce a preponderance of either male or female offspring. • competition for matings among individuals of one sex (usually males) can create variation in reproductive success: when competition is keen, some males may achieve many matings, others none. • it is often the largest males that win the lion's share of contests over access to females. • In mammals, a mother cares directly for her offspring, and her condition is likely to influence the fitness of her offspring. Therefore, females in poor condition should invest more in female offspring, which are likely to mate successfully regardless of the parental care they receive. Females in good condition ideally should produce male offspring, which will grow large and fare well in male-male competition for mates.
Female wood rats… • a laboratory study of female wood rats (Neotoma floridana). • when investigators restricted food intake during the first 3 weeks of lactation to below the maintenance level of a nonreproductive female, mothers actively rejected the attempts of male offspring to nurse. • As a result, males starved, and the sex ratio of the offspring at 3 weeks shifted to about one male for every two females. • Faced with the likelihood that their young would be poorly nourished and that some of them would probably die before they were weaned, the mothers favored their female offspring.
Sex ratio and pollution • Recent study: “Lower oxygen levels in polluted waters could lead to a higher ratio of male fish that may threaten certain species with extinction” • hypoxia (O2 depletion) can affect sex development, sex differentiation and the sex ratio in fish species. hypoxia can inhibit the activities of certain genes that control the production of sex hormones and sexual differentiation in embryonic zebra fish. • In his study, Wu found that 61 % of zebra fish - a universal freshwater fish widely used in scientific and pollution research - spawned into males under regular oxygen conditions. Under hypoxia conditions, the ratio of males increased to 75 %. • Hypoxia can be a naturally occurring phenomenon, particularly in areas where salt and fresh waters meet in estuaries such as the Pearl River Delta. It can also be caused by pollution.
Human sex ratio and pollution: PCBs… • PCBs were banned in the 1970s, … they are linked to problems with the brain, nervous and hormone systems, and although average levels in the human body have dropped, human exposure continues. Why? PCBs are persistent contaminants, which means they build up in the environment and in us. • Evidence continues to build that PCBs also affect birth sex. A recent study of blood serum from women who were pregnant in San Francisco in the '60s found that those with higher PCB levels were more likely to give birth to boys than those with low PBC levels.
