Genome size evolution in the class Mammalia

Abstract

In this study, intra- and interspecific genome size variation was assessed among species within the Order Chiroptera. Outgroup taxa from the Orders Dermoptera and Primates also were examined. This study additionally addressed the hypothetical plesiomorphic genome size of mammals by examination of genome sizes in Metatheria, the sister group to the Eutheria, and Xenarthra. Evolutionary patterns of genome size change along phylogenetic lineages of Chiroptera were inferred using the Hypothetical Taxonomic Unit (HTU) optimizing procedure of Farris (1970). This procedure was used to assign an optimal set of HTUs to dendrograms generated by clustering procedures. Using this procedure, it was possible to superimpose genome size data onto phylogenetic hypotheses and infer genome size changes along phylogenetic branches. The present study was designed to answer three questions. The first was whether genome sizes in chiropteran species were significantly smaller than the 7 pg value typical of mammals. Examination of this issue entailed a survey of genome sizes among 82 chiropteran species, and thirteen species from four other mammalian Orders. A side issue focused on genome size as a variable trait by examining the range of genome size variation among individuals within populations, among populations within species, and among species. The second question was whether the Order Chiroptera is monophyletic, i.e., are Megachiroptera and Microchiroptera sister taxa, or are Megachiroptera and Primates sister taxa? These competing hypotheses were tested by examining the genome size of representatives of both presumptive chiropteran suborders as well as representatives from the Grandorder Archonta (Primates and Dermoptera). The last question regarded the hypothetical primitive genome size of mammals. It has been hypothesized that the primitive genome size of mammals is 7 pg DNA (Kraemer et al., 1972). This hypothesis, however, was based primarily on commonality. In this study, genome sizes of metatherian and eutherian mammals were analyzed phylogenetically.