BIOL 203 Reproductive Strategies Lecture 09, Fall 2024 (PDF)

Lecture 09 Reproductive Strategies BIOL 203 October 23rd, 2024 1 Learning objectives 1. Describe both sexual and asexual reproduction 2. Explain how organisms can evolve as separate sexes or as hermaphrodites 3. Describe the typically balanced sex ratios o...

Lecture 09 Reproductive Strategies BIOL 203 October 23rd, 2024 1 Learning objectives 1. Describe both sexual and asexual reproduction 2. Explain how organisms can evolve as separate sexes or as hermaphrodites 3. Describe the typically balanced sex ratios of offspring and how this can be modified by natural selection 4. Explain how mating systems describe the pattern of mating between males and females 5. Explain how sexual selection favours traits that facilitate reproduction 2 Key Concept Reproduction can be sexual or asexual 3 Reproduction can be sexual or asexual All organisms reproduce In plants, animals, fungi, and protists, reproduction accomplished through: Sexual reproduction = progeny inherit DNA from two parents via union of two gametes Asexual reproduction = progeny inherit DNA from a single parent 4 Reproduction can be sexual or asexual Evolution of reproductive strategies involves a large variety of factors, many influenced by ecological conditions Sexual/asexual? Separate sexes/hermaphrodites? Sex ratio of offspring? Impacts on fitness? 5 Sexual reproduction Sexual reproduction = progeny inherit DNA from two parents via union of two gametes Mixes alleles (independent assortment, crossing over, random fertilization) Main source of genetic variation for most species E.g.) In humans, over 8 million possible combinations of chromosomes during meiosis (independent assortment) Results in over 70 trillion possible combinations of sperm/egg (random fertilization) 6 Asexual reproduction Asexual reproduction = progeny inherit DNA from a single parent/gamete Often, offspring are genetically identical to parent and to each other (clones) Only source of variation is mutations (very slow!) Two main types: Vegetative reproduction Parthenogenesis 7 Vegetative reproduction Vegetative reproduction is a form of asexual reproduction in which an individual is produced from the nonsexual tissues of a parent Produces clones = offspring genetically identical to parents E.g.) Walking fern (Asplenium rhizophyllum) 8 Parthenogenesis Some organisms can produce a viable embryo without fertilization = parthenogenesis Often arise from diploid eggs Evolved in plants, several groups of invertebrates, no mammals, very few lizards, amphibians, birds, and fish 9 Parthenogenesis Can produce either clones or genetically variable offspring Clones produced when germ cells develop into egg cells without meiosis Genetically variable offspring produced in two ways: Meiosis I occurs; meiosis II blocked Meiosis completes and two gametes fuse to form diploid egg 10 Costs of sexual reproduction Sexual reproduction comes with costs to the organism Sex organs require considerable energy/resources Mating can also be challenging; requires time/resources E.g.) plants produce flowers, courtship displays, finding mates 11 Costs of sexual reproduction Sexual reproduction can increase risk of mortality (e.g., herbivory, predation, parasitism, etc.) E.g.) kalutas (Antechinus spp.) Live for 11 months, then during 2 week period frantically search for females Single breeding event lasts up to 14 hours High testosterone suppresses immune system Ultimately kills male (what kind of parity is this?) 12 Costs of sexual reproduction For organisms in which an individual is either male or female, also have the cost of meiosis 50% reduction in the number of genes a parent passes to next generation vs asexual reproduction (100% DNA from one parent) Natural selection favours parents that contribute most copies of their genes to subsequent generations 13 Costs of sexual reproduction In hermaphrodites (organisms that can produce both sets of gametes), cost of meiosis is offset by mixing male gametes with one parent and female gametes with another Parental care can also offset cost of meiosis if more offspring can be raised 14 Benefits of sexual reproduction: Purging mutations Mutations occur in all organisms; some are harmful (deleterious) Through asexual reproduction, mutations are passed from one generation to the next and can accumulate (especially in clones) Through sexual reproduction, deleterious mutations can be purged by: Independent assortment – gamete may not receive deleterious allele Random fertilization – the two gametes that fuse may not have deleterious allele Homozygous offspring likely unviable, will not reproduce 15 Benefits of sexual reproduction: Purging mutations Since asexual reproduction leads to accumulation of deleterious mutations, should lead to reduced fitness and ultimately extinction Does it? Can compare species within a phylogeny Asexual reproduction should appear more recently than sexual reproduction (i.e., evolve from a sexually reproducing ancestor) 16 Benefits of sexual reproduction: Purging mutations E.g.) stick insects (Timema spp.) 17 Benefits of sexual reproduction: Purging mutations Does not fit all asexual species E.g.) Bdelloid rotifers >300 species, all asexual/female Maybe produce offspring faster than new deleterious mutations arise, so some always have nonmutated genotype = clonal selection 18 Benefits of sexual reproduction: Genetic variation and future environmental variation Sexual reproduction also produces greater genetic variation than asexual reproduction Environments changes over time/space; offspring likely to encounter different conditions than parents Greater variation increases probability of survival in new conditions 19 Benefits of sexual reproduction: Genetic variation and evolving parasites/pathogens Genetic variation also helpful for surviving pathogens Pathogens have shorter generation times and much larger population sizes compared to hosts → can evolve much faster than hosts, evade defences E.g.) chytrid fungus (Batrachochytrium dendrobatidis) 20 Benefits of sexual reproduction: Genetic variation and evolving parasites/pathogens Sexual reproduction increases probability of developing resistance to pathogen Evolutionary arms race between hosts and parasites = Red Queen hypothesis “Now, here, you see, it takes all the running you can do, to keep in the same place.” 21 Testing the Red Queen hypothesis E.g.) Potamopyrgus antipodarum (snail) and Microphallus (parasitic trematode) Red Queen hypothesis states that sexual reproduction in snail allows them to adapt fast enough to persist 22 Testing the Red Queen hypothesis Worm eggs are ingested by snail If the snail does not have resistance, parasite eggs hatch and form cysts in snail gonads Snail becomes sterile, population cannot persist 23 Testing the Red Queen hypothesis Ducks eat infected snails, pathogens mature in duck intestine, pathogen produces eggs Deposited into water when duck defecates Means that pathogen is most abundant where there are ducks (shallow waters) 24 Testing the Red Queen hypothesis Snail’s mode of reproduction depends on depth of water In shallow water, use sexual reproduction to continue to evolve resistance to pathogen (more variation) In deep water, use asexual reproduction (lower costs of reproduction; rapid reproduction) 25 Testing the Red Queen hypothesis E.g.) Round word (Caenorhabditis elegans) Raised asexual/sexual populations Exposed to bacterial parasite Measured survival over 30 generations 26 Concept check What are the benefits of asexual reproduction? What are the costs of sexual reproduction? How does genetic variation help a population survive pathogens? 27  Sexually reproducing organisms can evolve Key Concept to have separate sexes or be hermaprodites 28 Sexually reproducing can evolve to have separate sexes or to be hermaphrodites Wide array of sexual strategies for male and female function Most vertebrates have separate sexes, whereas most plants are hermaphrodites Simultaneous hermaphrodites = both male/female structures exist at same time (many molluscs, worms, plants) Sequential hermaphrodites = individual has one sexual function then switches to the other (some molluscs, echinoderms, fishes, plants) 29 Sexually reproducing can evolve to have separate sexes or to be hermaphrodites Plants arrange sexual structures in several ways Hermaphroditic plants possess male/female structures within same flower = perfect flower 30 Sexually reproducing can evolve to have separate sexes or to be hermaphrodites Some plants have separate male/female flowers on same plant = monoecious Plants with separate male/female flowers on separate plants = dioecious 31  HFitness = SSFitness Comparing strategies The reproductive strategy with the highest fitness should be favoured by natural selection If female can invest in male function to gain lots of male fitness and lose only a little female fitness, hermaphrodite favoured Total fitness of hermaphrodite greater than either male/female 32  HFitness = SSFitness Comparing strategies If cost of adding second sexual function too high, separate sexes will be favoured Fitness of either male/female greater than fitness of hermaphrodite 33 Comparing strategies In plants with pollinators, flowers already present (largest cost), makes sense to produce both male/female reproductive parts (perfect flowers) >2/3 flowering plants produce perfect flowers 34 Comparing strategies In animals, complex gonads require lots of energy to make Also costs associated with attracting mates, defending territories, parental care, etc. Separate sexes favoured when actively seek mates, invest in brood care Hermaphrodism favoured for sedentary broadcast spawners (e.g. corals) 35 Selfing versus outcrossing of hermaphrodites Selfing/self-fertilization = individual uses its own male gametes to fertilize its own female gametes Can lead to inbreeding depression (reductions in fitness due to accumulation of deleterious alleles) Can avoid via outcrossing = breeding with other individuals Can also be genetic constraints (self-incompatibility genes) 36 Selfing versus outcrossing of hermaphrodites Can avoid selfing via sequential hermaphrodism E.g.) Blue-headed wrasse (Thalassoma bifasciatum) Functionally female when small adult, becomes male when larger 37 Selfing versus outcrossing of hermaphrodites Can temporarily delay fertility E.g.) Many plants release pollen before making stigma receptive to pollen (prevents self-pollination) 38 Mixed mating strategies Some hermaphrodites use a mixture of mating strategies When a mate can be found, prefer outcrossing When mates rare, will self-fertilize Usually does not produce as many viable offspring, but better than nothing If deleterious mutations have been purged, this can be beneficial (avoids cost of meiosis) 39 Mixed mating strategies Using a mixture of outcrossing/selfing can be a response to lack of resources E.g.) orange jewelweed Outcrossing more costly than selfing (need to make nectar for pollinators) Higher levels of herbivory lead to more selfing 40 Concept check What is the difference between simultaneous and sequential hermaphrodism? When a fitness increment of increased male function results in a large cost in fitness of female function, would a population evolve separate sexes or hermaphrodites? 41  Sex ratios of offspring Key Concept are typically balanced but can be modified by natural selection 42 Sex ratios of offspring are typically balanced but can be modified by natural selection With separate sexes, sex ratio for males to females is usually 1:1 In some cases, evolutionary forces favour a skewed sex ratio Both genetic and environmental factors 43 Mechanisms of sex determination In mammals, birds, and many other species, sex is determined by sex chromosomes In mammals, XY system: XX = female; XY = male In birds, ZW system: ZZ = male; ZW = female In insects, many different systems 44 Mechanisms of sex determination Many species of reptiles have environmental sex determination In turtles, eggs incubated at lower temperatures produce males while higher temperature produce females In alligators/lizards, reverse is generally true Temperature-dependent sex determination 45 Is temperature-dependent sex determination adaptive? Is temperature-dependent sex determination adaptive? If temperatures that produce each sex also lead to higher fitness of each sex, then it would be adaptive Is this the case? 46 Is temperature-dependent sex determination adaptive? E.g.) Jacky dragon (Amphibolurus muricatus) Females produced at low and high temps, both males/females at intermediate temps Used a hormone-inhibitor to produce males at high and low temps Measured fitness over 3 year period 47 Is temperature-dependent sex determination adaptive? Males incubated at intermediate temps fathered more offspring than males incubated at high/low temps Females incubated at high/low temps produce more eggs than females incubated at intermediate temps 48 Mechanisms of sex determination Social environment can also drive sex determination E.g.) blue-headed wrasse Starting as small female may be beneficial; don’t need to compete with larger, dominant males When male dies, largest female becomes new male 49 Offspring sex ratio Females can have a large influence on the sex ratios of her offspring In some mammals, females can control whether X or Y chromosome carrying sperm are allowed to fertilize eggs In some birds, can control fraction of eggs which receive Z or W chromosome In hymenopterans, female determines sex of offspring by whether or not she fertilizes eggs 50 Offspring sex ratio Some species can control sex ratio through selective abortion E.g.) Red deer (Cervus elaphus) Male offspring often larger at birth Sex ratio changes as mothers age 51 Offspring sex ratio Sex ratio also changes throughout the year for yearlings More resources in spring, more even sex ratio Limited resources in winter, virtually all females produced 52 Frequency-dependent selection If sex ratio becomes skewed, the less abundant sex experiences higher fitness (less competition for mates) Because of this, natural selection will favour individuals which produce offspring of the less abundant sex, since those offspring will have less competition Leads to a natural balancing of sex ratios back to ~1:1 Frequency-dependent selection 53 Impacts of human activities on sex ratio Humans can cause unnatural changes in sex ratios E.g.) Sockeye salmon (Onchorhyncus nerka) Males typically larger, but natural variation in difference between males and females Fishing regulations limit anglers to largest fish; inadvertently harvest more males than females 54 Highly skewed sex ratios Sometimes natural selection favours highly skewed sex ratios E.g.) Fig wasps (Pegoscapus assuetus) Enters flower to pollinate it, lays eggs inside fig, dies Eggs are 90% female (fertilized eggs = female, unfertilized = male Once they hatch, offspring mate with each other, males die, females go off to pollinate new flowers 55 Highly skewed sex ratios Why do we get this? Local mate competition = competition for mates in a limited area with only a few males required to fertilize all the females If only one female enters flower to lay eggs, the only males available are her sons One son can fertilize multiple daughters, and doesn’t matter which one, so no fitness advantage to producing lots of sons 56 Highly skewed sex ratios 57 Highly skewed sex ratios Since mother determines sex of offspring (fertilized vs unfertilized), can also change sex ratio if needed E.g.) Two female wasps enter a flower and lay eggs, make sense to produce more males to increase the chance that they breed 58 Concept check What is the difference between genetic and environmental sex determination? When a population is composed of two sexes, why does the rarer sex have a fitness advantage? How does local mate competition favour the production of female-biased sex ratios in offspring? 59  Mating systems Key Concept describe the pattern of mating between males and females 60 Mating systems describe the pattern of mating between males and females Gametes take different amounts of energy to make Sperm is cheap; eggs are expensive A female’s reproductive success depends on number of eggs and quality of mates A male’s reproductive success depends on how many females he can fertilize 61 Mating systems describe the pattern of mating between males and females Mating systems describe the number of mates each individual has and the permanence of the relationship Subject to natural selection; often the product of the ecological conditions 62 Mating systems describe the pattern of mating between males and females Four main types of mating system: promiscuity, polyandry/polygyny (polygamy), and monogamy 63 Promiscuity Promiscuity is a mating system in which individuals mate with multiple partners and do not create a lasting social bond Most common strategy in animals, universal in outcrossing plants Success affected by number of gametes produced (with wind/water dispersal), quality of sperm, ability to attract/compete for mates 64 Polygamy Polygamy is a mating system in which a single individual of one sex forms long-term social bonds and mates with more than one individual of the opposite sex Most often, males are the promiscuous sex = polygyny Sometimes, females are the promiscuous sex = polyandry 65 Polygyny Polygyny is a mating system in which a male mates with many females Evolves when males compete for females, and females are picky Also occurs when a male can defend a group of females or can control access to resources attractive to females 66 Polyandry Polyandry is a mating system in which a single female breeds with multiple males Commonly occurs when the female is searching for genetically superior sperm or receives material benefits from males she mates with E.g.) Some butterflies produce spermatophores 67 Monogamy Monogamy is a mating system in which a social bond between one male and one female persists through rearing offspring Can be seasonal or lifelong Favoured when males provide parental care ~90% of birds since males can contribute equally to raising chicks