Plant sex chromosomes: molecular structure and function

Abstract

Recent molecular and genomic studies carried out in a number of model dioecious plant species, including Asparagus officinalis, Carica papaya, Silene latifolia, Rumex acetosa and Marchantia polymorpha, have shed light on the molecular structure of both homomorphic and heteromorphic sex chromosomes, and also on the gene functions they have maintained since their evolution from a pair of autosomes. The molecular structure of sex chromosomes in species from different plant families represents the evolutionary pathway followed by sex chromosomes during their evolution. The degree of Y chromosome degeneration that accompanies the suppression of recombination between the Xs and Ys differs among species. The primitive Ys of A. officinalis and C. papaya have only diverged from their homomorphic Xs in a short male-specific and non-recombining region (MSY), while the heteromorphic Ys of S. latifolia, R. acetosa and M. polymorpha have diverged from their respective Xs. As in the Y chromosomes of mammals and Drosophila, the accumulation of repetitive DNA, including both transposable elements and satellite DNA, has played an important role in the divergence and size enlargement of plant Ys, and consequently in reducing gene density. Nevertheless, the degeneration process in plants does not appear to have reached the Y-linked genes. Although a low gene density has been found in the sequenced Y chromosome of M. polymorpha, most of its genes are essential and are expressed in the vegetative and reproductive organs in both male and females. Similarly, most of the Y-linked genes that have been isolated and characterized up to now in S. latifolia are housekeeping genes that have X-linked homologues, and are therefore expressed in both males and females. Only one of them seems to be degenerate with respect to its homologous region in the X. Sequence analysis of larger regions in the homomorphic X and Y chromosomes of papaya and asparagus, and also in the heteromorphic sex chromosomes of S. latifolia and R. acetosa, will reveal the degenerative changes that the Y-linked gene functions have experienced during sex chromosome evolution.

Step-by-step evolution of neo-sex chromosomes in geographical populations of wild silkmoths, Samia cynthia ssp.

“…Moreover, recent findings in Drosophila suggest that the Y chromosome has an important role in male fitness and also contributes to adaptive phenotypic variation through a regulatory role of Y-linked polymorphic elements in gene expression (Lemos et al, 2008). In contrast, a gradual degeneration appears to be the primary driving force in evolutionarily young Y chromosomes as shown in the plant models Carica papaya and Silene latifolia (Jamilena et al, 2008). In addition, once the degenerated Y chromosome fuses or translocates to an autosome, a new cycle of its evolution starts.…”

Junk DNA promotes sex chromosome evolution

“…However, plant sex chromosomes have been found from moss to flowering plants, including familiar crop species, such as asparagus, hop, kiwi fruit, papaya and spinach (Matsunaga and Kawano, 2001). The ancient Y chromosome in the liverwort Marchantia polymorpha is small and largely heterochromatic (Yamato et al, 2007), whereas most Y chromosomes in flowering plants are the largest chromosomes in male genomes and many plant sex chromosomes are morphologically indistinguishable (Matsunaga, 2006;Jamilena et al, 2008). Why do sex chromosomes in flowering plants seem to retain their primitive characteristics similar to a pair of autosomes?…”

Karyotype and RAPD Analysis of Male and Female <i>Coccinia grandis</i> L. from Bangladesh

“…The large size Y-chromosome indicated the possibility of accumulation of repeated sequences, which is a sign of evolution of plant Y-chromosomes (Jamilena et al 2008, Bergero et al 2008, Heslop-Harrison and Schwarzacher 2011, Mariotti et al 2009). Moreover, Sousa et al (2013) reported that the subterminal repetitive sequences may have accumulated on the Y-chromosomes.…”