39. Sex determination and differentiation

Definitions

• Sex Determination: The biological process that establishes whether an individual will develop as male, female, or both. This is determined at the time of fertilization and is influenced by genetic or environmental factors.
• Sex Differentiation: The process through which the undifferentiated gonads and reproductive structures develop into either male or female organs and secondary sexual characteristics, depending on the initial sex determination.

Types of Sex Determination

A. Genetic Sex Determination (GSD)

• Based on the inheritance of specific sex chromosomes during fertilization.
• In humans:
• XX = Female
• XY = Male
• The presence of the Y chromosome and the SRY gene is crucial for initiating male development.
• Common in mammals, birds, insects, and many plants.

B. Environmental Sex Determination (ESD)

• Sex is determined by environmental conditions experienced during embryonic development.
• Examples:
• Temperature-dependent sex determination (TSD) in reptiles:
• Alligator eggs incubated at lower temperatures develop as females; higher temperatures produce males.
• Chemical cues or hormones can influence sex in certain fish and amphibians.

C. Cytoplasmic Sex Determination

• Sex is influenced by cytoplasmic factors like endosymbiotic bacteria.
• Example:
• Wolbachia infection in insects can:
• Induce parthenogenesis (females reproduce without males)
• Feminize genetic males
• Kill male embryos to skew population sex ratio

D. Hermaphroditism

• An individual has both male and female reproductive organs.
• Types:
• Simultaneous hermaphrodites: Both sets of gonads functional at the same time (e.g., earthworms, some snails).
• Sequential hermaphrodites:
• Protandry: Male first, then female (e.g., clownfish).
• Protogyny: Female first, then male (e.g., wrasse fish).