Lecture 22 Notes - Vandan Desai BIOL 442Developmental...

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Vandan Desai BIOL 442—Developmental Biology 1 Lecture #22—Sex Determination I. Sex Determination Is Not Always Permanent or Binary :: A. Sex determination—process by which an organism acquires sexual characteristics B. Sex determination is not always permanent (Ex: tropical clownfish): i. All clownfish are born males ii. Clownfish group consists of dominant male & female and 0-4 juvenile males iii. When the female dies , the dominant male becomes the dominant female and one of the juveniles becomes the dominant male C. Sex is not always binary (Ex: sexual phenotypes in plainfin midshipman fish ( Porichthys notatus) -- i. Female = small body size; only short vocalizations; no nesting/care of young; lays eggs in nests of Type I males ii. Type I Male = large body size; vocalize (“hum”) to attract mate and defend territory; excavate and defend nest; cares for eggs/hatchings; mates by courting and attracting females iii. Type II Male = small body size; only short vocalizations; no nesting/care of young; huge testis; two mating strategies: (1) “sneak spawning” —enters nest of Type I male and fertilizes egg, and (2) “satellite spawning” —ejaculates just outside the nest and fans sperm into the nest II. Primary vs. Secondary Sex Determination :: A. Primary vs. secondary sex determination— i. Primary sex determination—a tissue or cell directly assesses its sex ( cell autonomous ) Cells that undergo primary sex determination Flies = all cells of the body Mammals = gonadal cells (testes & ovaries) Primary sex determination is determined by chromosomes and sometimes by the environment ii. Secondary sex determination—a tissue or cell receives information about sexual identity from another tissue ( non-cell autonomous ) B. Most Drosophila cells undergo primary sex determination— i. Sex is determined within each individual Drosophila cell by the ratio of X chromosomes to autosomes ii. This mode of primary sex determination is demonstrated by the generation of Gynandromorphic flies (one side is one sex and other side is different sex)
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Vandan Desai BIOL 442—Developmental Biology 2
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Vandan Desai BIOL 442—Developmental Biology 3 v. Different roles for DSX M and DSX F Dsx (a TF) = controls many aspects of sexual dimorphism Dsx mutants = exhibit an intersexual phenotype DSX M = activates the male pathway & represses the female pathway DSX F = represses the male pathway & activates the female pathway C. Primary sex determination switches in mammals— i. Karyotype vs. phenotype in humans = Not all genes on X CH (Barr Body) is inactivated ; some remain active
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Vandan Desai BIOL 442—Developmental Biology 4 Therefore, sex is determined by the Y chromosome Hypothesis:
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