Thus far, two genetic mechanisms of sex determination have been empirically supported in the Hymenoptera: complementary chromosomal system of sex determination in ants and bees in Montana determination CSD  —  and maternal control sex determination, although the latter mechanism has only been documented for the parasitoid Nasonia and its preponderance among other haplodiploid species remains to be determined.
Homomorphic sex chromosomes: sex chromosomes that are morphologically indistinguishable. In plants, females and males often differ in flower morphology and abundance D awson and G eberand, although sex differences are often minor outside the flowers or inflorescencesthey do exist D awson and G eber ; E ppley and W enk Even when two taxa both have XY systems, they may not have evolved from a single ancestral XY system—a new GSD system could have re-evolved in a lineage after being lost.
Open in a separate window. In contrast to the lability of sex determination mechanisms in some groups, eutherian mammals, birds and many insects exhibit virtually no variation in how sex is determined Figure 3. View Article Google Scholar
Valenzuela N Multivariate expression analysis of the gene network underlying sexual development in turtle embryos with temperature-dependent and genotypic sex determination. The evolution of completely separate sexes, where individuals cannot give rise to both sperm and egg descendants, is thought to be fairly derived and is found primarily among multicellular organisms with rare unicellular exceptions e.
In fact, sex determination is a rapidly evolving trait in many lineages Figure 3and sometimes closely related species, or populations of the same species, have different modes of sex determination .
This fact has been used to explain the evolution of eusociality in many hymenopterans. Further extensions of the MAT region, possibly involving additional sex-antagonistic genes, led to the U and V chromosomes characterizing male and female gametophytes, as found, e.
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For example, natural inbreeding and the corresponding mating systems are incompatible with sl-CSD, because this would dramatically increase the production of diploid males  —  ,  — . Karaiskou, E. Identification of a sex-determining region in Nile tilapia Oreochromis niloticus using bulked segregant analysis.
In the most extreme cases, the Y or W is entirely lost, resulting in so-called X0 and Z0 systems.