Invasions of sex-determining genes are facilitated when the new sex-determining gene or a gene closely linked to it has beneficial effects on fitness . PLoS Genet 7: e Megapodes had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex.
Matson, C. Discussion With the availability of molecular techniques, we may now hope to understand more about how sex chromosomes evolve. In this variant of the XY system, females have two copies of the sex chromosome XX but males have only one X0. Pronounced differences of recombination activity at the sex determination locus SDL of the honey bee, a locus under strong balancing selection.
Molecular genetics of plant Y chromosomes Our understanding of the evolution of plant sex chromosomes and sex genetically controlled sex determining mechanisms in Tampa should be advanced by the use of molecular markers, so several groups are searching for these.
Contrary to Myth 1, however, diverse mechanisms are used to determine sex  Figure 3Figure 4 ; Box 2. Likewise, cellular endosymbionts are only transmitted through the mother and can create maternally inherited female-biased sex ratios; examples include male-killing bacteria in butterflies and Drosophila .
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. This is followed by a summary of more recent findings on the underlying molecular genetics of sex determination Myth 2 revisited and a deconstruction of common misconceptions of sex chromosome evolution in humans and other species Myth 3 revisited.
At least one monotremethe platypuspresents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds and lacks the SRY gene. Restriction of recombination followed by gene loss on the Y has resulted in the morphological differentiation of sex chromosomes for a review of the molecular and evolutionary processes involved in Y degeneration, see genetically controlled sex determining mechanisms in Tampa.
Charlesworth B The evolution of chromosomal sex genetically controlled sex determining mechanisms in Tampa and dosage compensation.
Graves A full understanding of the diversity of sex determination mechanisms will require that we expand the taxonomic breadth of study systems well beyond classic model organisms. Sry in mammals, Sxl in D. There are also some species that are only one sex due to parthenogenesis , the act of a female reproducing without fertilization.
Second, some taxa may have few genes under sexually antagonistic selection on their sex chromosomes and thus avoid selection to suppress recombination between the X and Y .