In many animals and some plants, however, the switch to develop into a female or male does not lie in the genes. PLoS Biol 12 7 : e In the Leydig cells, Sf1 activates the genes encoding the enzymes that make testosterone. Many biologists are habituated to thinking about sex determination through the familiar examples of mammals and D.
In these cats, the fur pigmentation gene is X-linked; one X-chromosome encodes an orange coat colour and the other gives a black or tabby.
However, the epithelium soon produces a new set of sex cords, which do not penetrate deeply into the mesenchyme, but stay near the outer surface cortex of the organ. Polygenic sex determination: sex is determined by multiple genes e. Testosterone and dihydrotestosterone Although testosterone is one of the two primary masculinizing hormones, there is evidence that it might not be the active masculinizing hormone in certain tissues.
In rodents, for example, many species with unusual sex-determining systems can be found: XY females in chromosomal sex determination in animals in Calgary lemming species, X0 females or XX males in vole species, and X0 females and males in some Japanese spiny rats and mole voles . Bardoni and her colleagues proposed that this region contains a gene for a protein that competes with the SRY factor and that is important in directing the development of the ovary.
These myths do not survive a survey of sex determination systems across the tree of life. Direct evidence for the trade-off hypothesis is sparse and observations consistent with it come from hermaphroditic great pond snails, which reallocate resources to female function when sperm production is experimentally abolished and from strawberries, in which increased pollen production comes at the cost of reduced seed set .
Haplodiploidy: males develop from unfertilized, haploid eggs, and females from fertilized, diploid eggs e.
Int J Dev Biol 46 : 75— Saumitou-Laprade P, Cuguen J, Vernet P Cytoplasmic male sterility in plants: molecular evidence and the nucleocytoplasmic conflict. For example, male-sterility genes have been identified in rice, with at least one male-sterility gene found on each of rice's 12 chromosomes—hence each autosome could, in principle, evolve a sex-determining function .
Methods Sexual system, karyotype, genome size, ploidy, and life history data were collated from taxonomic accounts, books on flora and fauna, online scientific databases, and the primary scientific literature sources from the literature were preferred when available.