Abstract
The structure of a genome can be described at its simplest by the number of chromosomes and the sex chromosome system it contains. Despite over a century of study, the evolution of genome structure on this scale remains recalcitrant to broad generalizations that can be applied across clades. To address this issue, we have assembled a dataset of 823 karyotypes from the insect group Polyneoptera. This group contains orders with a range of variations in chromosome number, and offer the opportunity to explore the possible causes of these differences. We have analysed these data using both phylogenetic and taxonomic approaches. Our analysis allows us to assess the importance of rates of evolution, phylogenetic history, sex chromosome systems, parthenogenesis and genome size on variation in chromosome number within clades. We find that fusions play a key role in the origin of new sex chromosomes, and that orders exhibit striking differences in rates of fusions, fissions and polyploidy. Our results suggest that the difficulty in finding consistent rules that govern evolution at this scale may be due to the presence of many interacting forces that can lead to variation among groups.
Highlights
Chromosome number is one of the fundamental characteristics of a genome
Reduction in chromosome number can happen through Robertsonian translocations with the loss of non-essential DNA [2] or happen through the fusion of two chromosomes at the telomeres followed by loss of one of the centromeres [3,4]
We assembled a trait dataset of chromosome number, sex chromosome system (SCS), genome size and reproductive mode. We analysed these data in both a taxonomic and a phylogenetic framework to determine the impact of the sexual system on rates of chromosome number evolution, the source of transitions in SCSs, and identify differences in patterns of chromosome number evolution among orders
Summary
Chromosome number is one of the fundamental characteristics of a genome. It is the first information collected about most genomes. We assembled a trait dataset of chromosome number, sex chromosome system (SCS), genome size and reproductive mode We analysed these data in both a taxonomic and a phylogenetic framework to determine the impact of the sexual system on rates of chromosome number evolution, the source of transitions in SCSs, and identify differences in patterns of chromosome number evolution among orders. We use the mean of this value calculated across the posterior distribution of trees We evaluated both an absolute tip rate and a directional tip rate (accounting for whether the change is an increase or decrease in chromosome number). We fit both standard and phylogenetically corrected linear models where genome size predicted either the absolute rate or the directional rate [36,37]. Data and all R code for analyses are provided in a GitHub repository: https://github.com/Tsylvester8/Polyneoptera
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