Dosage deficiencies at X-linked loci thus select for an up-regulation of X genes in males to restore the proper balance in expression ratios between autosomal and X-linked genes 42, In the silkworm, B. View Article Google Scholar 7.
Despite numerous theoretical predictions for how and why sex determination mechanisms change, many hypotheses remain untested. Intermediate gametes do worse than small ones in terms of mobility and numbers, and worse than large ones in terms of provisioning. 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 .
In turtles and crocodiles, sex is determined by the temperature of the surrounding environment at a specific period in the development of a fertilized egg. In human sexual reproduction, two distinct gametes fuse to form a zygote. Sex Dev 4: — Sex as a threshold trait has been inferred in several other fish  —  and invertebrates and further examples of multiple interacting loci controlling sex determination are no doubt waiting to be described.
One partial solution is the spatial separation of male and female gonads in the same individual, as in monoecious plants with separate male sex chromosomes and sex determination invertebrates in Berkeley female flowers e.
Gavrilov I. Genetic conflict and evolution of mammalian X-chromosome inactivation. This group has approximately described species, and along with Raphidioptera was formerly considered part of Neuroptera. Sexual differentiation humans Development of the reproductive system gonads Mesonephric duct Paramesonephric duct.
View Article Google Scholar 5. In Drosophilathe absence of crossing over in males ensures that such a neo-Y chromosome is genetically isolated from its partner Comp Biochem Physiol —
There are approximately species of stick insects. Figure 5. The bubble insert graph for the plant clades represents the relative proportion of species with documented sex chromosomes within plants with separate sexes.