In nature, there are different types of conserved patterns of sex-determining mechanisms which range from male heterogamety (XY) found in the majority of mammals and female heterogamety (ZW) found in most birds to environmental sex determination (ESD) expressed most commonly as temperature sex determination (TSD) found in reptiles and some fish species .
The number of fish species that present male heterogamety compared to female heterogamety is higher .
The independent origins of multiple sex chromosomes via centric fusion involving a cryptic Y chromosome in the widely divergent Megupsilon and Garmanella suggests that cryptic male heterogamety
(XY) might have characterized the most-recent common ancestor of the Cyprinodontina.
niloticus is proposed to display male heterogamety
, in contrast to female heterogamety in O.
Gender determination by heterogamety, XY (male heterogamety
) or ZW (female heterogamety), is a well-known system involving 2 genetic factors (Yusa 2007).
Production of all female and sterile coho salmon, and experimental evidence for male heterogamety
Pacific salmon have a genotypic sex-determining system with male heterogamety
; the male is XY and the female is XX (7).
Multiple-chromosomal sex determining systems and male heterogamety
have been documented for several other cypridid species including sexual populations of C.
The data show that the male hybrid unfitness is found in the five taxa with male heterogamety
. Assuming independence, the probability of this close an agreement between the two characters is [(5/7).sup.5][center dot][(2/7).sup.2] = 0.0152.
We compared taxa with female heterogamety to their nearest closest relatives with male heterogamety, and asked in which of those sister taxa is female hybrid inviability more common.
There is some suggestion that male heterogamety in this group is derived [Hillis and Green 1990] and that male hybrids have lower fertility [Spurway 1953].