User:Hafiza Sara Akram/Plant Sex Chromosomes

Sex chromosomes are pair of chromosomes carrying genes of sex determination. The sex chromosome of plants are almost similar to animals. They can vary in size and shape in different plant species. Most of the parts of the sex homologous chromosomes remain non recombinant. They are unknown in lycophytes, relatively common in vascular plants and most common in bryophytes. Majority of plants have male heterogamety (XY males, XX females), plant self fertilization ability is the important genetic evidence of male heterozygocity. Individuals in most flowering plant species are either monoecious or dioecious, both male and female flowers on same plant and male or female functions in different plants respectively.

Differentiation:
Silene latifolia is a short lived, dioecious and perenial herbaceous specie with chromosomal male sex determination. It has been extensively studied and used as model plant species for sex chromosome structure and evolution. In Silene latifolia each cell contains 12 pairs of chromosomes (24, XX in female and 24, XY in male). This plant has degenerated Y-chromosome quite larger than X-chromosome. X-chromosome is always present in ovary (female reproductive part) while either X and Y-chromosome can be present in male reproductive organ called stamen. There is a small homologous region in X and Y-chromosome called pseudoautosomal region. Histone modification spread throughout the sex chromosomes except pseudoautosomal region. Y-chromosomes has less supressive histone modifications compare to X-chromsomes. The active histone modification are also enriched in some part of male X-chromosome and only in one choromosome of female.

Sex determination in plants:
There are two loci carried by Y-chromosome which involved in sex determination. One locus has role in suppression of female organ development and other locus is responsible for male organ development. There are some reported genes WUSCHEL (WUS), SHOOTMERISTEMLESS (STM), CUPSHAPED COTYLEDON1 (CUC1) and CUC2 show different expression pattern in male and female. These genes involved in sex determiantion in S. latifolia.

Sequence Composition and Evolution:
Sex chromosomes originating from pair of ordinary autosomes, have evolved in plants. Amplification of transposable elements, tandem repeats especially accumulation of long tandem repeats (LTR) retrotransposones, are responsible for plant sex chromosome evolution. The insertion of retrotransposones is probably the major cause of y-chromosome expansion and plant genome size evolution. Retrotransposones contribute in size determination of sex chromosomes and its proliferation varies even in closely related species. LTR and tandom repeats play dominant role in the evolution of S. latifolia sex chromosomes. Athila is new family of retroelements, discoverd in Arabidopsis thaliana, present in heterochromatin region only. Athila retroelements overrepresented in X but absent in Y, while tandem repeats are enrich in Y-chromosome. Some chloroplast sequences have also been identified in the Y-chromosome of ''S. latifolia. S. vulgaris has more retroelements in their sex chromosomes compare to S. latifolia''. Microsatellite data shows that there is no significant difference between X and Y-chromosome microsatellites in both Silene species. This would conclude that microsatellites do not participate in Y-chromosome evolution. The portion of Y-chromosome that never recombine with X-chromosome faces selection reduction. This reduced selection leads to insertion of transposable elements and accumulation of deleterious mutation. The Y become larger and smaller than X due to insertion of retroelement and deletion of genetic material respectively. The genus Humulus is also used as model for the study of sex chromosomes evolution. Based on the phylogenetic topology ditribution there are three regions on sex chromosomes. One region that stops recombining in the ancestor of H. lupulus, second that stops recombining in modern H. lupulus and the third region called pseudoautosomal region. H. lupulus is the rare case in plants in which Y is smaller than X, while its ancestor plant has the same size of both X and Y chromosomes. This size difference between X and Y was peviously believed to be due to the deletion of genetic material in Y, however, it is due to the duplication or retrotransposition in X chromosome.