9 Autosomes

1. Chromosome Evolution In Coleoptera Heath Blackmon and Jeff Demuth University of Texas at Arlington Introduction Changes in the number or the arrangement of chromosomes could have important biological consequences. With over 360,000 described species and considerable cytogenetic variability beetles provide the resource to test this question. Coleoptera accounts for 40% of all insects and for 25% of all living species (Crowson 1981). A long history of phylogenetic research has yielded countless lower level morphological phylogenies, and the recent explosion of sequence data has allowed the production of robust molecular based phylogenies down to the subfamily level (Hunt et al. 2007). Beetles have diverse autosomal numbers and sex chromosomes that can be easily categorized. Finally, beetles have been widely studied cytogenetically providing thousands of karyotypes for analysis. Distribution of Sex Chromosomes The major sex chromosome systems have been observed in both major suborders but they are distributed differently in the two suborders. Polyphagan species are observed to have Xyp sex determination system 61% of the time while among the Adephagans XO and XY are both more common 39% and 40% respectively. Of particular interest is the increased frequency at which the y chromosomes is lost in the suborder Adephaga when compared to Polyphaga. Within Adephaga the y is lost in 39% of all species including members of the families Carabidae, Dytiscidae, Gyrinidae, Haliplidae, and Trachypachidae, While in the suborder Polyphaga very large and well sampled families like Curculionidae do not show a single loss of the y chromosome despite having otherwise variable karyotypes with multiple types of sex chromosome pairing and even numbers of sex chromosomes within single genera. This is suggestive of a systemic change in the behavior of the sex chromosomes. Variation in Number of Chromosomes The number of chromosomes in Coleoptera karyotypes is highly variable. The order has a male diploid number which ranges from a low in the Elatrid Chalcolepidius zonatus 2n=4 to a high in the Carabid Dixus capito obscuroides with 2n=69. The presence of nine autosomes in the one sampled Myxophagan species Ytu zeus and 10 total chromosomes in the haplodiploid Archostematan Micromalthus debilis are indicative of an ancestral state of 9 autosomes for the order Coleoptera. The suborder Adephaga departs from this with a mode of 18 autosomes. However, nine autosomes have been recorded once in both Dytiscidae and Haliplidae as well as in all 4 of the sampled Noteridae. More investigation is required to determine if the doubling of the autosomal number is due to a series of fissions or an ancient polyploidization. Survey of Karyotype Data A database of Coleoptera karyotypes was compiled from previously published data. We recorded multiple karyotypes for a single species if reports either differed in the karyotype data or reported data from a new geographic regions. Because of improvements in microscopy we also recorded recent findings in favor of those reported before 1930. This process produced a total of 4,547 records. This is a significant increase compared to the last compilation of Coleoptera karyotype data which included 2160 karyotypes (Smith and Virkki 1978). 9 Autosomes Coleoptera Karyotypes Karyotypes are usually produced from testes squashes, and are reported as meioformula. This takes the form of the number of autosomes followed by the sex chromosome system and its pairing type as well as the presence or absence of supernumerary chromosomes. These four features are the primary data that we targeted in our compilation. 18 Autosomes 9IIXyp Number of autosomes & their arrangement Sex Chromosome Pairing Sex Chromosome System Coleoptera Sex Chromosomes The sex chromosomes of Coleoptera are highly heteromorphic and often will not form synaptonemal complexes. The accurate pairing of the sex chromosomes has been accomplished in a number of ways throughout the order. The Xyp pairing is the most common in the order and is found in the closely related order Megaloptera adding support for the idea that it might be the ancestral condition for the order. From Citations Crowson RA. 1981. The biology of the Coleoptera. New York: Academic Press. Hunt T, Bergsten J, Levkanicova Z, Papadopoulou A, John OS, Wild R, Hammond PM, Ahrens D, Balke M, Caterino MS. 2007. A comprehensive phylogeny of beetles reveals the evolutionary origins of a superradiation. Science 318(5858):1913. Maddison, D. R. and W. P. Maddison, 2005. MacClade 4: Analysis of phylogeny and character evolution. Version 4.08a. http://macclade.org. Smith SG, Virkki N, John B. 1978. Animal cytogenetics / Vol. 3, Insecta. 5, Coleoptera. Berlin: Gebrüder Borntraeger. Phylogenetic tree extending to the genus level representing 322 clades. At the family level we based the tree’s topology on the molecular phylogeny published by Hunt et al in 2007. At lower levels the tree is assembled from a variety of published morphological and molecular based phylogenies. Ancestral states were reconstructed using maximum parsimony within MacClade. Dermestidae Trachypachidae Melandryidae Coccinellidae Stenotrachelidae Hydrophilidae Curculionidae Scarabaeidae Staphylinidae Tenebrionidae Anthicidae Cicindelinae Buprestidae Chrysomelidae Bostrichidae Cerambycidae Anthribidae Appioninae Attelabidae Trogossitidae Elateridae Passalidae Lampyridae Silvanidae Dytiscidae Scolytidae Histeridae Byrrhidae Bruchinae Anobiidae Erotylidae Haliplidae Cantharidae Melyridae Carabidae Brentidae Nitidulidae Lucanidae Silphidae Meloidae Alleculinae Trogidae Gyrinidae Cleridae Ciidae Lycidae