Structure and evolution of the Y-chromosomal and mitochondrial DNA of cattle

Abstract

The research described in this thesis is focused on the structure and evolution of the bovine Y-chromosome and the use of paternal markers in molecular diagnostics. The Y-chromosome has emerged together with the X-chromosome early during the evolution of the mammals by differentiation of a pair of autosomes. The Y-chromosome is now the male chromosome and is transmitted from father to son. It starts the development of male properties in mammals primarily by the action of the gene SRY. Several other genes on the Y-chromosome are involved in spermatogenesis. The first chapter is a general introduction and starts out with a description of the well characterized human Y-chromosome, which may serve as a model for its mammalian homologues. Genetic polymorphisms on the Y-chromosome have become useful as markers of the paternal lineage for tracing descent and inheritance. The potential use of these markers is illustrated by the test methods described in chapter 2 which consists of three parts. In chapter 2a, the development of two complementary methods is described: a mitochondrial PCR-RFLP and satellite fragment length polymorphism or SFLP for the detection and differentiation of the species origin of beef. Chapter 2b describes assays based on mutations in the paternally inherited Y-chromosomes of different bovine species for the detection of male introgression. In chapter 2c we describe a real time PCR method for the sensitive detection of an interspersed genomic element which is a suitable target for DNA detection of ruminant material in feedstuff. In Chapter 3 we have used the mitochondrial and Y-chromosomal variation for elucidating the phylogeny of the bovinii or cattle-like species. As expected, we found a clear relationship of ox and zebu, but also of gaur, gayal and banteng and of yak and American bison. However, we found that the mitochondrial DNAs from American bison and wisent (European bison) were quite different, although their hybrid offspring is fully fertile and their Y-chromosomal sequences are clearly related. Thus it appears that wisent has via the paternal lineage another origin than via the maternal lineage. We have contributed to the detailed study of the bovine Y-chromosome by an analysis of the multicopy TSPY gene family in chapter 4. Comparison of active TSPY copies of different bovine species yielded evidence for concerted evolution, since TSPY copies of taurine cattle were more similar to each other than to copies of other species. We predict that other Y-chromosomal repetitive elements, genes as well as high-copy non-coding sequences, are also subject to rapid homogenization. In this respect, the Y-chromosomal repeats resemble the autosomal centromeric satellites.