LECTURE 2
GENETIC AND ENVIRONMENTAL SEX DETERMINATION
I. Introduction
a. What defines male and female?
Male= capacity to produce sperm (spermatogenesis)--possessing testes
Female=capacity to produce ovum (oogenesis)--possessing ovaries
b. Two ways sex is determined in vertebrates
Genetic sex determination (by genes)
Environmental sex determination (not by genes but by environmental influences)
II. Genetic sex determination
a. Sex chromosomes
Heterogametic and homogametic chromosomes
-heterogametic -- if the pair of sex chromosomes is different (e.g., XY)
-homogametic -- if the pair of sex chromosomes is same (e.g., XX)
-XX/XY (if male is heterogametic)--mammals, amphibians, some fishes
-ZZ/ZW (if female is heterogametic)--reptiles, birds, some fishes
b. Mechanism of sex determination by genes
Male-determining genes
-SRY (Sex-determining Region of the Y chromosome)--on Y chromosome
*codes for a protein TDF (testis-determining factor)
*HMG (high mobility group) protein that binds and bends DNA
*required for testes formation in mammals
XX female + SRY-->XX' male
-SOX9 gene--on autosome
*also codes for a HMG protein
*appears to be activated by SRY
*required for testes formation in ALL vertebrates
XX female + Xtra copy of SOX9-->XX' male
-SF1 gene--on autosome
*a transcription factor
*required for formation of bipotential gonads and maintenance of testes once formed
XX male - SF1 --->abnormally formed testis (can't make testosterone)
Female-determing genes
-Dax 1--on X chromosome
*a transcription factor (orphan receptor)
*required for formation of ovaries
*antagonizes the action of SRY and SF1
XY male + Xtra copies of Dax 1 --> X'Y female (duplicated X)
-Wnt4--on autosome
*a transcription factor
*required for formation of ovaries
*inhibited by SRY in males
XX female -Wnt4 -->abnormal ovaries with male features (make male proteins and hormones)
c. Abnormal nondisjuction of sex chromosomes
Abnormality in humans
-XXY (Klinefelter's syndrome)--male, mental retardation, sterile, female features
-XO (Turner's syndrome)--female, no mature gonad, sexually infantile
-XXX (superfemale)--superfemale, usually sterile
Regular features in other animals retained through evolution
-XXY male and XXXX females in spiny anteaters
-XO male and XO female in European mole rat and Japanese spiny rat and some other rodents. One X has SRY (male) and one doesn't (female)
-XY females in some Eurasian wood lemmings
*X-linked gene inhibits testes formation--leading to more females than males in the population
III. Environmental sex determination
a. Temperature-dependent sex determination
Mechanism
-very complex. Mostly involves synthesis of estrogen and receptors at a particular temperature.
Temperature F -->female gonad
Temperature M -->male gonad
Intermediate temperature-->50/50 male/female
-probably an estrogen-driven event.
Some examples
-turtles
*lower temperature = males
*in green sea turtles, position of eggs in the clutch determines the sex (center warmest)
-crocodiles and alligators
*lower temperature = female
*choice of nest is very important and can skew sex ratio up to 10:1
-lizards (one giving birth to live youngs)
*Southern water skink
*bask to regulate body temperature to produce 1:1 sex ratio
-some fishes and amphibians (less known)
Implications
-endocrine disruptors
*many act as estrogen mimics
*DDT, dicofol, phytoestrogens (from soy and legumes)
*can alter sex ratio-->population dynamics-->extinction
*can also cause abnormal gonadal development
-global warming
*higher long-term temperature results in skewed sex ratio
b. Social factors
Sex-changing fish
-Australian cleaner wrasses
*lives in groups of 1 male:6 females
*remove male-->1 female becomes male
*female-->male (protogynous)
-Anemone fish
*one monogamous breeding pair in colony (rest nonbreeding)
*remove breeding female-->breeding male becomes breeding female, and nonbreeding males becomes mature
*male-->female (protandrous)