Transcript document
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 Having a 2 of the same Chromosome? Edmund Beecher Wilson & Nettie Stevens 1905 chromosomal XY sex-determination system 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 Lyonisation • Occurs only in females • X chromosome has fur color control gene • 2 alleles – Black & Orange • White results from genetic condition of unpigmented hair What About… • Genotypic sex determination – Birds & mammals • Homogametic – Humans female • Heterogametic – Human male – Bird female Temperature-dependent Sex Determination • Temperature eggs exposed to during development determines biological sex of offspring • Thermosensitive period during development Turtles • Males produced typically at lower incubation temperatures – 22.5-27C • Females higher temperatures – 30C females • Lizards and crocodiles opposite pattern American Alligator • Females result from eggs incubated at both high and low temps • Males develop at intermediate temperatures 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 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) • 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 Definitions of Male & Female • Gender is a social categorization • Biological categories have to apply to all forms of life – Algae, humans, sea stars, plants Biological Definition • Based on gamete size • Male – Small gametes – Sperm (spermatozoa) • Female – Large gametes – Egg (oocyte) Isogamy • Gametes same size Mating types • • • • Characteristic of isogametic species Differences in cell surfaces Algae, fungi, protozoans No male/ female Clamydomonas sp 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 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 Why Two Sexes • Cytoplasm control • Energy investment strategies 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 Evolution of Anisogamy • Isogamy ancestral condition • Anisogamy independently evolved many times • Driving force is energy investment – Resulted in disruptive selection 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 The Scenario • In a population diversity in gamete size exists (bell shaped curve) • Some individuals make small nutrient poor gamete • Some make an inbetween gamete sizes • Some make large nutrient rich gametes Small Nutrient Poor Gamete • Proto-male • Pro – can produce lots of cheap gametes – Increases number of reproductive events • Greater reproductive potential – Greater fitness & more individuals in the population • Con – 2 nutrient poor gametes together reduce survival Large Nutrient Rich Gametes • Proto-female • Pro – Offspring produced have greater probability of survival • Con – Produce fewer gametes – Fewer reproductive events – less prevalent in the population Some make an in-between gamete sizes • No selective advantage • Gamete not big enough to gain selective advantage of having lots of nutrients • Gametes not produced in large enough numbers to increase reproductive success Proto-Male Strategy • Once gametes begin to diverge anisogamy itself creates pressure that advanced differentiation • Protomales produced in large numbers • Success is limited by ability to find rarer protofemales • Selection forces act on Proto-male to increase adaptations for finding proto-female Proto-Female Strategy • Reproductive success driven by producing fewer high quality gametes Consequences • Males – Lots of gametes – Little energy per gamete produced – Evolution favors mechanisms for encountering female – Adaptations • Motile • Locate- egg tracking systems • Females – Fewer gametes – Lots energy per gamete produced – Evolution favors mechanisms for being encountered – Adaptations • Chemical attractants • Selection Coevolution of Males & Females Male vs Female 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 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 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.” 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 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 Consequence of Anisogamy • Male – Increased proportion of reproductive effort directed into mating effort • Female – Increased proportion of reproductive effort directed into Parental effort 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 Effects • Results in morphological differences • Results in behavioral differences Isogamous Species Anisogamous Species Sources • http://www.sciencedaily.com/releases/201 2/03/120315145415.htm