Factors that effect chromosomal evolution: repetitive DNA in conservative versus rapidly evolving karyotypes

Chromosomal evolution has been used in many systematic and evolutionary investigations, including speciation mechanisms, rates of speciation, predicting effective long-term population sizes, fixation of chromosomal rearrangements, gene duplication, sex determination, and hybrid zones. However, little is known about the cause and effect relationship of chromosomal evolution. Ideas to explain this relationship include demographic models, negative heterosis, recombinational breakdown, and molecular effects.

In a study of equids, considered to be chromosomally the most rapidly evolving group of mammals, the hypothesis that tandemly repeated DNA sequences may be responsible for chromosomal evolution was proposed. Predictions of this hypothesis for rapidly evolving genomes include: 1) an abundance of tandemly repeated sequences, 2) these sequences will have undergone intragenomic movement, and 3) these tandemly repeated sequences will represent multiple classes of elements.

In this study, I test this hypothesis by examining repetitive sequences in a bat species (Macrotus waterhousii) which has a conservatively evolving karyotype. If M.- waterhousii has fewer numbers and classes of tandemly repeated sequences and if these sequences are restricted to certain chromosomal fields, then the hypothesis remains viable.

A genome library was constructed for M.- waterhousii and 649 clones, representing 0.1% of the genome, were screened using Artibeus jamaicensis as an outgroup. This phylogenetic screening process produced 11 hypervariable clones. These were separated into three classes: a) clones found in either the ingroup or the outgroup but not in both; b) clones which possessed different numbers of hybridizing bands in the ingroup versus the outgroup; and c) clones which possess differences in copy number of the ingroup versus the outgroup. These clones were sorted into two families based on cross-hybridization experiments. One family contained only one clone; whereas, the second family contained the remaining 10 clones. Hybridization of this single clone to genomic DNAs of A- jamaicensis and M.. waterhousii digested with four restriction enzymes showed it to be tandemly repeated in Ajamaicensis. The remaining clones produced smears, indicative of interspersed distribution.

In situ hybridization of these clones to chromosomes of R. waterhousii and A- jamaicensis revealed hybridization to four clones. Two clones, produced hybridization to centromeric regions on two pair of chromosomes and slight hybridization to some of the remaining chromosomes in A- jamaicensis but no visible hybridization to M.. waterhousii. Two additional clones showed faint hybridization to most chromosomes in M.- waterhousii and A- jamaicensis indicating high copy number with an interspersed pattern of distribution.

Comparison of data from the chromosomally conserved bat species to equids which have rapidly evolving karyotypes (possessing 30 hypervariable clones, seven families, and intragenomic movement of sequences) suggests that the conservatively evolving bat species have fewer copies and types of hypervariable sequences than do equids. It is concluded that the hypothesis that a molecular basis, specifically rapidly evolving repetitive sequences, may play an important role in chromosomal evolution merits further testing.