Abstract
Despite the fundamental role of centromeres two different types are observed across plants and animals. Monocentric chromosomes possess a single region that function as the centromere while in holocentric chromosomes centromere activity is spread across the entire chromosome. Proper segregation may fail in species with monocentric chromosomes after a fusion or fission, which may lead to chromosomes with no centromere or multiple centromeres. In contrast, species with holocentric chromosomes should still be able to safely segregate chromosomes after fusion or fission. This along with the observation of high chromosome number in some holocentric clades has led to the hypothesis that holocentricity leads to higher rates of chromosome number evolution. To test for differences in rates of chromosome number evolution between these systems, we analyzed data from 4,393 species of insects in a phylogenetic framework. We found that insect orders exhibit striking differences in rates of fissions, fusions, and polyploidy. However, across all insects we found no evidence that holocentric clades have higher rates of fissions, fusions, or polyploidy than monocentric clades. Our results suggest that holocentricity alone does not lead to higher rates of chromosome number changes. Instead, we suggest that other co-evolving traits must explain striking differences between clades.
Highlights
Chromosome number stability is generally expected among lineages as shifts in chromosome number can lead to a decrease in fitness [1,2,3]
Monocentric chromosomes must have a single region that functions as the centromere, while in holocentric chromosomes, centromere activity is present across the entire chromosome
We found that Lepidoptera, a holocentric clade, exhibits some of the highest rates of chromosome number evolution, while other holocentric clades exhibit some of the lowest rates
Summary
Chromosome number stability is generally expected among lineages as shifts in chromosome number can lead to a decrease in fitness [1,2,3]. This stability in chromosome number is thought to be driven by the underdominance of chromosomal rearrangements [4]. This expected stability is challenged by some clades that exhibit striking variation in chromosome number as well as the interdigitation of fast and slow evolving lineages within clades [5,6,7]. In light of the potential impacts of chromosomal change, identifying traits associated with increased rates of chromosomal rearrangements is a key step in understanding patterns of extant diversity
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