Molecular evolutionary genetics of Macropodinae: Phylogenetic resolution of the Macropus genus and analysis of centromere composition

Date of Completion

January 2007


Biology, Genetics




This thesis endeavors to demonstrate a link between centromere composition and chromosome evolution. The Macropodinae marsupials (wallabies and kangaroos) are uniquely amenable to such study because they have experienced a recent species radiation accompanied by broad karyotypic divergence. To determine whether centromere sequence composition follows the phylogenetic history of species evolution or follows patterns of convergent breakpoint reuse through chromosome evolution, we examined the phylogenetic trajectory of the three representative centromere probes isolated from Macropus rufogriseus.^ The constitution of the centromeric portions of the sex chromosomes of the red-necked wallaby, Macropus rufogriseus (family Macropodidae), was first investigated to develop an overview of the sequence composition of centromeres within a single marsupial genome. Sequence and cytogenetic analyses of three representative clones showed that they localize specifically to centromeric heterochromatin, yet differ in copy number and intrachromosomal distribution. Immunocytohistochemistry assays defined a restricted, point localization of the functional centromere within the enlarged, pericentric and heterochromatic regions of the M. rufogriseus centromeres. This study identified constituents of the M. rufogriseus chromosomes and developed a more comprehensive overview of the composition of a marsupial centromere.^ In the second portion of the study the three M. rufogriseus centromere probes were localized to nine species within the Macropus genus. The localization of the three probes was compared with the phylogenetic history of a mitochondrial gene, Cyt b, a nuclear gene, TRSP, and the chromosomal histories of the syntenic blocks that define the different karyotype arrangements. Convergent contraction or expansion of predominant centromere satellites was found to accompany specific karyotype rearrangements. The phylogenetic history of these centromere sequences displayed the convergence of centromere composition in divergent species through convergent breakpoint reuse between syntenic blocks. These data support the "library hypothesis" of centromere evolution within this genus as each species possess the same satellites yet has experienced differential expansion and contraction of these satellites. Thus, we have identified a correlation between the evolution of centromere satellite sequences, the reuse of syntenic break points, and karyotype convergence in the context of a gene-based phylogeny. ^