Environmental sex reversal, Trojan sex genes, and sex ratio adjustment: conditions and population consequences

Abstract

The great diversity of sex determination mechanisms in animals and plants ranges from genetic sex determination (GSD, e.g. mammals, birds, and most dioecious plants) to environmental sex determination (ESD, e.g. many reptiles) and includes a mixture of both, for example when an individual's genetically determined sex is environmentally reversed during ontogeny (ESR, environmental sex reversal, e.g. many fish and amphibia). ESD and ESR can lead to widely varying and unstable population sex ratios. Populations exposed to conditions such as endocrine-active substances or temperature shifts may decline over time due to skewed sex ratios, a scenario that may become increasingly relevant with greater anthropogenic interference on watercourses. Continuous exposure of populations to factors causing ESR could lead to the extinction of genetic sex factors and may render a population dependent on the environmental factors that induce the sex change. However, ESR also presents opportunities for population management, especially if the Y or W chromosome is not, or not severely, degenerated. This seems to be the case in many amphibians and fish. Population growth or decline in such species can potentially be controlled through the introduction of so-called Trojan sex genes carriers, individuals that possess sex chromosomes or genes opposite from what their phenotype predicts. Here, we review the conditions for ESR, its prevalence in natural populations, the resulting physiological and reproductive consequences, and how these may become instrumental for population management.

Epigenetic modification and inheritance in sexual reversal of fish

“…In organisms with GSD (often based on sex chromosomes), the primary sex of an individual is determined at the moment of fertilization by heritable genetic elements that differ between the sexes, whereas organisms with ESD do not possess a primary sex at fertilization and have their sex induced during ontogeny (Bull 1983;Stelkens and Wedekind 2010;Matsumoto and Crews 2012). Generally, the sex of GSD species will be fixed for life, but in some GSD species, the primary sex can be altered during development, such that individuals can develop into the opposite sex without changing their genotype.…”

“…Generally, the sex of GSD species will be fixed for life, but in some GSD species, the primary sex can be altered during development, such that individuals can develop into the opposite sex without changing their genotype. This phenomenon is known as environmental sex reversal (ESR) (Stelkens and Wedekind 2010) and is relatively common in insects (Vance 1996;Narita et al 2007), fishes (Devlin and Nagahama 2002), amphibians (Wallace et al 1999), and reptiles (Quinn et al 2007). Diverse environmental triggers for ESR have been documented, including abiotic (e.g., temperature, pH, hormones) and biotic (e.g., crowding, pathogens) factors, of which temperature is the most broadly studied (Kato et al 2011).…”

“…Diverse environmental triggers for ESR have been documented, including abiotic (e.g., temperature, pH, hormones) and biotic (e.g., crowding, pathogens) factors, of which temperature is the most broadly studied (Kato et al 2011). Currently, artificial induction of sex reversal is widely used in aquaculture by exposure of fish to exogenous steroids or different rearing temperatures in order to produce the preferred sex (Stelkens and Wedekind 2010). ESD lacks the flexibility of ESR, but may be adaptive because it can preferentially produce the sex that benefits most from the best possible hatching environment in terms of future reproductive success (Janzen 1995;Shine 2005, 2008).…”

“…ESD lacks the flexibility of ESR, but may be adaptive because it can preferentially produce the sex that benefits most from the best possible hatching environment in terms of future reproductive success (Janzen 1995;Shine 2005, 2008). However, a common concern is that ESD systems may become a burden under rapid environmental change, such as global warming and ocean acidification, because it might result in nonadaptive sex ratios that are skewed toward predominantly male or female offspring (Ospina-Alvarez and Piferrer 2008;Mitchell and Janzen 2010;Stelkens and Wedekind 2010). Understanding the mechanisms by which genotype and environment interact to control early sex determination is therefore both of fundamental interest and of relevance for domesticated livestock and natural population conservation.…”

SUHU INKUBASI MEMPENGARUHI LEVEL EKSPRESI SOX9 PADA TAHAP AWAL PERKEMBANGAN GONAD EMBRIO Chelonia mydas (PENYU HIJAU)

“…Rasio seks anakan pada taksa tersebut dapat berubah drastis akibat pergeseran suhu inkubasi meskipun hanya sebesar 1°C saja [4]. Karena rasio seks merupakan salah satu parameter demografik yang penting untuk viabilitas populasi, maka pemahaman yang lebih baik terhadap peran lingkungan dalam mempengaruhi rasio seks jelas sangat dibutuhkan [5].…”