From Digenic to Monogenic Sex Determination in Insects: A Genetic Model Based on Imprinting and X Chromosome...

Background/Objectives: In digenic populations, all females produce males and females in their offspring. Monogenic populations are composed of gynogenic (female-producing) and androgenic (male-producing) females. A theoretical population genetic model for evolution of digenic to monogenic populations is presented here. Methods: A controlling gene was associated with each of the four processes that characterise monogenic populations: (1) oogenesis is conventional, whereas spermatogenesis is unusual and it is characterised by the exclusive formation of X-bearing sperm (gene (s)), i.e., the paternal chromosomes are eliminated so that only the maternal ones are transmitted to the next generation; (2) the X chromosome that is eliminated in the zygote is the one inherited from the father (gene r); (3) an imprinting process occurs in the mother (gene g), which protects the maternally inherited X chromosome from elimination in the zygote and the whole maternal chromosome complement in spermatogenesis; (4) a maternal factor is produced during oogenesis (gene e), which inactivates the elimination factor [r] in the zygote, thus controlling the elimination of the paternal X chromosome. The sequences of emergence of the genes (e s r g) that transform a digenic population into a monogenic one were analysed. Results: The following evolutionary sequences were found: (1) the sequence (r s e) under dominant conditions of gene (s) and recessive conditions of gene (r); and (2) the sequences (s r e), (r s e), and (e s r) under recessive conditions of gene (s) and gene (r). It was also found that the process of genomic imprinting is a necessary condition for the generation of a monogenic population. Furthermore, a quantitative change in the interaction between the elimination factor and its maternal inhibitor modifies the genotypic formula of the monogenic state. Conclusions: The number and types of evolutionary transitions of a digenic to a monogenic population depends on the dominant or recessive characteristic of the newly emerging genes. The imprinting process must already be present in the digenic population from which the monogenic one evolves; otherwise, the population cannot reach the monogenic state.