Sex chromosomes do not appear suddenly in animals or plants. rather, it is the pair of autosomes bearing the sex determination genes that have evolved specialized features, i.e., the degeneration of the Y chromosome, resulting in heteromorphy that became the hallmark of sex chromosomes. Two major sex chromosome systems have evolved. One is the active Y chromosome system, or the XY system (or ZW in a heterogametic female system), in which females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The other is the X-to-autosome (A) balance system, in which the ratio of X:A chromosomes determines sex by an X chromosome counting system and the Y chromosome is dispensable.....
the hypothesis that angiosperms originated from a hermaphroditic ancestor is supported by the prevalence of hermaphrodites among about 72% of all species (Takhtajan, 1969).
diverse mechanisms that evolved to promote outcrossing include temporal separation of the maturation of male or female organs, gametophytic or sporophytic self-incompatibility, structural avoidance, monoecy, and ultimately dioecy.
the first step toward sex chromosome evolution is the occurrence of male sterile or female sterile mutations leading to the development of unisexual flowers,,, such mutations occur frequently and repeatedly in plant species, as demonstrated by the presence of unisexual species in 75% of angiosperm families and numerous male sterile mutants in domesticated crop plants (Yampolsky and Yampolsky, 1922 ; Renner and Ricklefs, 1995). also, stamen and carpel development involve large numbers of genes at various developmental stages, and mutations of any of the many regulatory genes could trigger abortion or loss of function of male and/or female organs (Wellmer et al., 2004),,,,,
in dioecious flowers, male sterile mutation can occur from the early stages of the first appearance of the stamen primordia until the late stages of microspore formation; likewise, female sterility can also occur from early stages of inception of the carpel until the late stage of the formation of microspores (Matsunaga and Kawano, 2001).
unisexual species have sex determination mechanisms ranging from genetic to environmental, but genetic factors play far greater roles. Genetic mechanisms range from a single locus on an autosome to heteromorphic sex chromosomes containing multiple genes involved in sex determination. In some unisexual species, such as hemp, cucumber, and maize, male or female sterility is influenced by environmental factors such as light intensity, day length, temperature, mineral nutrition, and plant hormones (Frankel and Galun, 1977 ; Chailakhyan, 1979 )
cytoplasmic male sterility caused by mutations on the mitochondrial genome is widely used in production of crops that benefit greatly from heterosis, but only nuclear male sterility can trigger sex chromosome evolution...
in plants it is possible to follow both forward and reverse evolution between dioecism and hermaphroditism because dioecious mutants from hermaphrodites and hermaphrodite mutants from the dioecious plants are each occasionally found,,,
different plant species at various stages of sex chromosome evolution provide a time series for examining the forces driving Y chromosome degeneration.
according to the Mulles Rachet Theory the Y used to be an X,,,,its just lost a leg
the Y is the shape it is because once recombination is suppressed around the sex determination region, an incipient Y chromosome starts to differentiate by accumulating deleterious mutations, transposable element insertions, chromosomal rearrangements, and selection for male-specific alleles,,,,
some plant species have recently evolved homomorphic sex chromosomes near the inception of this evolutionary process, while a few other species have sufficiently diverged heteromorphic sex chromosomes.
,,,,
