We review some recently published results on sex chromosomes in a diversity of species. We focus on several fish and some plants whose sex chromosomes appear to be 'young', as only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. However, the age of these systems is not yet very clear. Even without knowing what proportions of their genes are genetically degenerate, these cases are of great interest, as they may offer opportunities to study in detail how sex chromosomes evolve. In particular, we review evidence that recombination suppression occurs progressively in evolutionarily independent cases, suggesting that selection drives loss of recombination over increasingly large regions. We discuss how selection during the period when a chromosome is adapting to its role as a Y chromosome might drive such a process.

/pdf/steps-in-the-evolution-of-heteromorphic-sex-chromosomes-29whtyn0tu.pdf

Steps in the evolution of heteromorphic sex chromosomes

Recent advances in understanding of male infertility have implicated two major causative factors, oxidative stress and Y chromosome deletions. A major cause of oxidative stress appears to be the high rate of reactive oxygen species generation associated with the retention of excess residual cytoplasm in the sperm midpiece. Other possible causes include the redox cycling of xenobiotics, and antioxidant depletion or apoptosis. Oxidative stress induces peroxidative damage in the sperm plasma membrane and DNA damage in both the mitochondrial and nuclear genomes. Nuclear DNA damage in the germ line of the father may be associated with pathology in the offspring, including childhood cancer and infertility. Gene deletions on the non-recombining region of the Y chromosome account for the infertility observed in about 15% of patients with azoospermia and 5-10% of subjects with severe oligozoospermia. The Y chromosome is particularly susceptible to gene deletions because of the inability of the haploid genome to deploy recombination repair in retrieving lost genetic information. Aberrant recombination, defective chromatin packaging, abortive apoptosis and oxidative stress may all be involved in the aetiology of DNA damage in the germ line. The factors responsible for Y chromosome deletions in spermatozoa remain unresolved but may be one facet of a central reproductive problem: controlling the amount of oxidative stress experienced by germ cells during their differentiation and maturation in the male reproductive tract.

/pdf/oxidative-stress-dna-damage-and-the-y-chromosome-3dr9oq8vcx.pdf

Oxidative stress, DNA damage and the Y chromosome.

The human Y chromosome is intriguing not only because it harbours the master-switch gene that determines gender but also because of its unusual evolutionary history. The Y chromosome evolved from an autosome, and its evolution has been characterized by massive gene decay. Recent whole-genome and transcriptome analyses of Y chromosomes in humans and other primates, in Drosophila species and in plants have shed light on the current gene content of the Y chromosome, its origins and its long-term fate. Furthermore, comparative analysis of young and old Y chromosomes has given further insights into the evolutionary and molecular forces triggering Y-chromosome degeneration and into the evolutionary destiny of the Y chromosome.

/pdf/y-chromosome-evolution-emerging-insights-into-processes-of-y-1dlkj6vagj.pdf

Y-chromosome evolution: emerging insights into processes of Y-chromosome degeneration

http://aerg.canberra.edu.au/library/sex_general/2006_Graves_sex_chromosome_specialization_and_degeneration_review.pdf

Sex chromosome specialization and degeneration in mammals.

A systematic search of the nonrecombining region of the human Y chromosome (NRY) identified 12 novel genes or families, 10 with full-length complementary DNA sequences. All 12 genes, and six of eight NRY genes or families previously isolated by less systematic means, fell into two classes. Genes in the first group were expressed in many organs; these housekeeping genes have X homologs that escape X inactivation. The second group, consisting of Y-chromosomal gene families expressed specifically in testes, may account for infertility among men with Y deletions. The coherence of the NRY's gene content contrasts with the apparently haphazard content of most eukaryotic chromosomes.

Functional coherence of the human y chromosome

Mapping of human X-borne genes in distantly related mammals has defined a conserved region shared by the X chromosome in all three extant mammalian groups, plus a region that was recently added to the eutherian X but is still autosomal in marsupials and monotremes Using comparative mapping of human Y-borne genes, we now directly show that the eutherian Y is also composed of a conserved and an added region which contains most of the ubiquitously expressed Y-borne genes Little of the ancient conserved region remains, and the human Y chromosome is largely derived from the added region

B Duffy

Papers

The human Y chromosome derives largely from a single autosomal region added to the sex chromosomes 80-130 million years ago.