Sex determination in vertebrates usually follows two paths: genotypic (GSD), where sex is set by chromosomes at fertilization, and environmental. Amphibians, including frogs, display remarkable flexibility, often blending these mechanisms. This diversity allows certain species to alter their sexual development in response to external or internal cues. The ability of West African frogs to change gender points to this unusual reproductive plasticity.
Sex Reversal in West African Frog Species
Yes, certain West African frog species exhibit the ability to undergo sex reversal, though the circumstances vary between natural and laboratory observations. One highly studied example involves the African Clawed Frog, Xenopus laevis, a species native to the region and widely used in scientific research. In controlled laboratory settings, Xenopus laevis has been shown to undergo complete functional sex reversal when exposed to specific chemicals during its larval stage.
This phenomenon means that a frog with the genetic makeup of a male can develop into a female that is fully capable of reproduction. The existence of natural sex change, known as sequential hermaphroditism, has also been suggested in another species, the Common Reed Frog (Hyperolius viridiflavus). Observations on captive colonies of this reed frog indicated that some females underwent a change to become functional males after successfully breeding.
While the observation in the reed frog is not widely accepted as a common natural occurrence, it provides an example of female-to-male sex change (protogyny). The African Clawed Frog demonstrates male-to-female reversal under environmental stress, highlighting the inherent developmental plasticity of these species. These specific types of changes confirm that gender change is a biological possibility within West African frog populations.
Triggers for Gender Change
The ability of these frogs to change gender is rooted in the interplay between genetics, hormones, and the environment. The underlying mechanism involves specific steroids, such as estrogen, which directs gonadal development toward the female sex. The enzyme responsible for producing estrogen is aromatase (CYP19), which converts male hormones (androgens) into female hormones (estrogens).
In amphibians, the expression of the aromatase enzyme in the developing gonads is a major factor in determining whether the individual develops ovaries or testes. In Xenopus laevis, for instance, a gene called dm-w triggers female differentiation by inducing the expression of aromatase. A shift in the balance of these internal chemical signals can therefore override the frog’s genetic blueprint.
External environmental factors are potent triggers for this shift. Temperature-dependent sex determination (TSD) is observed in some amphibians, where temperature during a sensitive developmental period determines the sex of the offspring. Another trigger is the presence of endocrine-disrupting chemicals (EDCs) in the environment, often from agricultural or urban runoff. Chemicals like the herbicide atrazine induce aromatase activity in Xenopus laevis genetic males, leading to the complete feminization of the tadpole.
Implications of Reproductive Plasticity
This reproductive plasticity carries implications for the survival of wild populations, especially when environmental conditions are rapidly changing. The ability to change sex offers an evolutionary advantage, particularly in small or isolated populations where one sex may become scarce. This flexibility allows the population to quickly rebalance its sex ratio, maximizing the chances for successful breeding.
However, human activity is manipulating this system with potentially harmful consequences. The presence of EDCs in aquatic environments triggers sex reversal, leading to severely skewed sex ratios. If a high percentage of genetic males are feminized, the population suffers a shortage of reproductively capable males. When a sex-reversed female (a genetic male) mates, she is only capable of producing male-genotype offspring. This cycle rapidly decreases the number of genetic females in the next generation, threatening the species’ long-term viability.