Evolutionary dynamics of autosomal-heterosomal rearrangements in a multiple-X chromosome system of tiger beetles (Cicindelidae)

<p>Abstract</p> <p>Background</p> <p>Genetic systems involving multiple X chromosomes have arisen repeatedly in sexually reproducing animals. Tiger beetles (Cicindelidae) exhibit a phylogenetically ancient multiple-X system typically consisting of 2–4 X chromosomes and a single Y. Because recombination rates are suppressed in sex chromosomes, changes in their numbers and movement of genes between sex chromosomes and autosomes, could have important consequences for gene evolution and rates of speciation induced by these rearrangements. However, it remains unclear how frequent these rearrangements are and which genes are affected.</p> <p>Results</p> <p>Karyotype analyses were performed for a total of 26 North American species in the highly diverse genus <it>Cicindela</it>, tallying the number of X chromosomes and autosomes during mitosis and meiosis. The chromosomal location of the ribosomal rRNA gene cluster (rDNA) was used as an easily scored marker for genic turnover between sex chromosomes or autosomes. The findings were assessed in the light of a recent phylogenetic analysis of the group. While autosome numbers remained constant throughout the lineage, sex chromosome numbers varied. The predominant karyotype was n = 9+X₁X₂X₃Y which was also inferred to be the ancestral state, with several changes to X₁X₂Y and X₁X₂X₃X₄Y confined to phylogenetically isolated species. The total (haploid) numbers of rDNA clusters varied between two, three, and six (in one exceptional case), and clusters were localized either on the autosomes, the sex chromosomes, or both. Transitions in rDNA localization and in numbers of rDNA clusters varied independently of each other, and also independently of changes in sex chromosome numbers.</p> <p>Conclusion</p> <p>Changes of X chromosome numbers and transposition of the rDNA locus (and presumably other genes) between autosomes and sex chromosomes in <it>Cicindela </it>occur frequently, and are likely to be the result of fusions or fissions between X chromosomes, rather than between sex chromosomes and autosomes. Yet, translocations between sex chromosomes and autosomes appear to be common, as indicated by the patterns of rDNA localization. Rearranged karyotypes involving multiple sex chromosomes would reduce recombination, and hybrid dysgenesis selects against polymorphic populations. Hence, the high frequency of these rearrangements could be a cause of the great species diversity in <it>Cicindela</it>.</p

Character optimization using parsimony was conducted separately for the number of sex chromosomes (2, 3, or 4 X chromosomes) and rDNA localization (localization on autosomes only, on the autosomes and X chromosomes, and on the heterosomes only)

<p><b>Copyright information:</b></p><p>Taken from "Evolutionary dynamics of autosomal-heterosomal rearrangements in a multiple-X chromosome system of tiger beetles (Cicindelidae)"</p><p>http://www.biomedcentral.com/1471-2148/7/158</p><p>BMC Evolutionary Biology 2007;7():158-158.</p><p>Published online 6 Sep 2007</p><p>PMCID:PMC2034538.</p><p></p> The tree is from Vogler et al. (2005) representing approx. 75% of the Nearctic species of (s.l.) and species names are preceded by an abbreviated subgenus label (CDIA, ; CELA, ; TRIB, ; PACH, ; HABR, ; CYLI, ; ELLI, ; BRAS, ). Several major groupings established in the previous phylogenetic analysis are indicated on the tree. Asterisks above the branches indicate nodes with good support (Bremer Support ≥3 and Bootstrap values ≥60%). Karyotype information is presented by symbols at the tips for all taxa represented in the current study. Different shading of branches indicates the character optimization for rDNA localization. Due to the incomplete taxon sampling it was not possible to assign the character changes to a precise branch deep in the tree, indicated by thin lines

Evolutionary dynamics of autosomal-heterosomal rearrangements in a multiple-X chromosome system of tiger beetles (Cicindelidae)-2

<p><b>Copyright information:</b></p><p>Taken from "Evolutionary dynamics of autosomal-heterosomal rearrangements in a multiple-X chromosome system of tiger beetles (Cicindelidae)"</p><p>http://www.biomedcentral.com/1471-2148/7/158</p><p>BMC Evolutionary Biology 2007;7():158-158.</p><p>Published online 6 Sep 2007</p><p>PMCID:PMC2034538.</p><p></p>chromosomes. . , diakinesis with n = 9 + XXXY showing two rDNA loci located on an autosomal pair. . , diakinesis with n = 9 + XXXY showing two rDNA loci located on an autosomal pair . , diakinesis with n = 9 + XXXY showing three rDNA loci, two located on an autosomal pair and one located on one of the 3 X chromosomes. . , metaphase I with n = 9 + XXY showing two rDNA loci located on an autosomal pair and one on the X chromosome. . , spermatogonial metaphase with 2n = 23 chromosomes showing six rDNA loci, one located on each of the four X chromosomes and two loci located on the Y chromosome. The white arrows indicate the heterosomal loci, the empty arrows indicate the autosomal loci. The arrowheads point to the sex chromosomes which are condensed in Fig. 1D forming the sex vesicle, and are recognizable individually forming the rosette-like structure in Figs. 1A, B, C, and E