User:Hafiza Sara Akram/Sex chromosome

Article body
Amplification of transposable elements, tandom repeats, especially accumulation of long tandom 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 size determination of sex chromosomes and its proliferation varies even in closely related species. Silene latifolia is a short lived, dioecious and perenial herbaceous specie with choromosmal sex determination. Its sex chromosome structure and evolution has been extensively studied. LTR and tandom repeats play dominant role in the evolution of S. latifolia sex chromosomes. Athila retroelements overepresented in X but absent in Y while tandom repeats enriched in Y-chromosome. Some chloroplast sequences have also been identified in Y-chromosome of S. latifolia. S. vulgaris has more retroelements in their sex chromosomes compare to S. latifolia. Microsatellite data shows that their is no significant difference between X and Y chromosomes microsatillites in both Silene species. This would conclude that microsatillite donot participate in Y-chromosome evolution. The portion of Y-chromosome that never recombine with X-chromosome faces selection reduction. This reduced selection lead to insertion of transposable elements and accumulation of deletrious mutation. The Y become larger and smaller than X due to insertion of retroelement and deletion of genetic materila respectively. The gens 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 stop recombining in ancestor of H. lupulus, second that stop recombining in modren H. lupulus and 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 same size of both X and Y chromosomes. This size difference should cause by deletion of genetic material in Y but that is not the case. This is because of complex dynamics like the large size of X than Y-chromosme may be due to duplication or retrotrnaspostion and size of Y remains same.