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Sex Differences

Sex Differences. Biological Sex. Recombination of genes Sexual reproduction involves multiple individuals Usually 2 Male vs Female What determines male and female?. In ye old days…. Aristotle Sex of infant determined by how much heat was in a man’s sperm.

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Sex Differences

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  1. Sex Differences

  2. Biological Sex • Recombination of genes • Sexual reproduction involves multiple individuals • Usually 2 • Male vs Female What determines male and female?

  3. In ye old days… • Aristotle • Sex of infant determined by how much heat was in a man’s sperm

  4. Having a 2 of the same Chromosome? Edmund Beecher Wilson & Nettie Stevens 1905 chromosomal XY sex-determination system

  5. Lyonisation • Mary Lyon 1962 • In Females each somatic cell has 2 X chromosomes • One X chromosome is inactivated • Random inactivation • Shortened and condensed • Females are genetic mosaics

  6. Lyonisation • Occurs only in females • X chromosome has fur color control gene • 2 alleles • Black & Orange • White results from genetic condition of unpigmented hair

  7. What About… • Genotypic sex determination • Birds & mammals • Homogametic • Humans female • Heterogametic • Human male • Bird female

  8. Temperature-dependent Sex Determination • Temperature eggs exposed to during development determines biological sex of offspring • Thermosensitive period during development

  9. Turtles • Males produced typically at lower incubation temperatures • 22.5-27C • Females higher temperatures • 30C females • Lizards and crocodiles opposite pattern

  10. American Alligator • Females result from eggs incubated at both high and low temps • Males develop at intermediate temperatures

  11. Genotypic Sex Determination • Humans exhibit heterogametic genotypic sex determination • All embryos have genes for both testes and ovary production • Female is the “Default” sex • Presence of a Y chromosome will stimulate the embryo to develop into a male offspring

  12. Genotypic Sex Determination • Sex determining region of Y chromosome • SRY gene complex • Gene expression of SRY gene will result in the production of Testes determining factor (TDF)

  13. Testes Determining Factor • TDF triggers activation of genes found on somatic chromosomes • ~7 weeks after fertilization • Activated genes responsible for the development of testes • Developing testes will produce Mullerian inhibiting substance • Causes regression of female gonads

  14. Definitions of Male & Female • Gender is a social categorization • Biological categories have to apply to all forms of life • Algae, humans, sea stars, plants

  15. Biological Definition • Based on gamete size • Male • Small gametes • Sperm (spermatozoa) • Female • Large gametes • Egg (oocyte)

  16. Isogamy • Gametes same size

  17. Mating types • Characteristic of isogametic species • Differences in cell surfaces • Algae, fungi, protozoans • No male/ female

  18. Clamydomonas sp

  19. Anisogamy • “not same gametes” • Gametes different size • Usually 2 sizes- small & large • Rarely some species have multiple sized gametes • Chlamydomonas euchlora • Divides 4-64 times any inbetween • 4 = big gametes • 64 = small gametes

  20. Why 2 sexes? • Multiple mating types can exist • Ex Slime molds..15 types • Fungi +/- • The magic number two • More than one ensures self does not breed with self • Produces variable offspring • Inbreeding avoidance • Smallest number possible to introduce diversity • Minimizes time invested finding mates

  21. Why Two Sexes • Cytoplasm control • Energy investment strategies

  22. Cytoplasm Control • Sperm evolved to prevent mixing of incompatible cytoplasm • Sperm very little other than DNA. Mitochondria & other organelles from sperm are typically excluded • Selection against incompatibilities in cytoplasm fusion drive evolution of a small & large gamete

  23. Evolution of Anisogamy • Isogamy ancestral condition • Anisogamy independently evolved many times • Driving force is energy investment • Resulted in disruptive selection

  24. Reproduction is an energy investment • Different strategies exist for reproductive investment • Trade-off exists between • size & number of gametes • Lot of small • Size & quality of gametes • Few high quality • Results in a selection force for 2 fertility strategies

  25. The Scenario • In a population diversity in gamete size exists (bell shaped curve) • Some individuals make small nutrient poor gamete • Some make an in-between gamete sizes • Some make large nutrient rich gametes

  26. Consequences • Males • Lots of cheap gametes • Little energy per gamete produced • Increases reproductive potential • Females • Fewer gametes lowers reproductive potential • Lots energy per gamete produced • Offspring produced have greater probability of survival • less prevalent in the population

  27. Adaptations • Males • Evolution favors mechanisms for encountering female • Adaptations • Motile • Locate- egg tracking systems • Females • Evolution favors mechanisms for being encountered • Adaptations • Chemical attractants • Selection

  28. Coevolution of Males & Females

  29. Male vs Female

  30. What happens when there is no hope…. • Fruit fly’s that mated repeatedly for several days no preference for alcohol • Males without access to females strongly preferred food mixed with alcohol • Alcohol satisfied desire for physical reward

  31. Bateman’s Principle Mated fruit fly’s in monogamous & promiscuous groups Male reproductive success increases with number of mates Female reproductive success does not increase with multiple mates

  32. Bateman’s Principle • Bateman 1948 • The sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete • Male strategy • Promiscuous • Female strategy • Choosy • Males should be eager, females… “The female, with the rarest exceptions, is less eager than the male…she is coy, and may often be seen endeavoring for a long time to escape.”

  33. Robert Trivers • 1972 • Expanded Bateman’s principle to extend beyond gamete scope to include all aspects of reproductive effort • Gestation, feeding, protection • The sex that invests more should be passive & discriminating

  34. Terminology • Reproductive effort • Proportion of the total energy budget of an organism that is devoted to reproduction • Parental Investment (effort) • Any investment made by parent to individual offspring to increase the survivability of the offspring • Mating effort • Any investment made in by individual to increase reproductive event

  35. Consequence of Anisogamy • Male • Increased proportion of reproductive effort directed into mating effort • Female • Increased proportion of reproductive effort directed into Parental effort

  36. Sex Conflict • Conflict of interest as two sexes reproductive strategies change over the course of evolution • Ex sponges sex strategies similar among sexes • Vs Peacock & peahen

  37. Effects • Results in morphological differences • Results in behavioral differences

  38. Isogamous Species

  39. Anisogamous Species

  40. Sources • http://www.sciencedaily.com/releases/2012/03/120315145415.htm

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