The evolution of eusociality is one of the major transitions in evolution, but the underlying genomic changes are unknown. We compared the genomes of 10 bee species that vary in social complexity, representing multiple independent transitions in social evolution, and report three major findings. First, many important genes show evidence of neutral evolution as a consequence of relaxed selection with increasing social complexity. Second, there is no single road map to eusociality; independent evolutionary transitions in sociality have independent genetic underpinnings. Third, though clearly independent in detail, these transitions do have similar general features, including an increase in constrained protein evolution accompanied by increases in the potential for gene regulation and decreases in diversity and abundance of transposable elements. Eusociality may arise through different mechanisms each time, but would likely always involve an increase in the complexity of gene networks.

Insect genome size diversity remains poorly sampled, with sparse and sporadic sampling of a few select orders and with many orders unrepresented or underrepresented in the genome size database. Here, we present 134 genome size estimates for 18 orders, including the first ever genome size estimates for eight orders, 38 families, 102 genera, and 131 species. Also reported here are three insect species genome size estimates that are corrections for unpublished genome size values that made it into the literature, including the smallest arthropod genome of the two spot spider mite (1C = 91 Mb). These estimates range from 91 to 7,752 Mb and provide a broader picture of genome size variation within Insecta and among all Arthropods. Proposed developmental constraints for holometabolous insect genome sizes are supported, with the majority of the species examined falling well under the hypothesized 1,978 Mb (2 pg) limit. The only exceptions occur in the highly diverse beetles (Coleoptera) (154 < 1C < 2,578) and the Mecoptera (1,890 < 1C < 2,134). Hemimetabolous orders include species with larger genomes. Of these, the Orthopteran genomes remain the largest (675 < 1C < 7,752). Hemipteran genomes are smaller (407 < 1C < 1,230), but with a very notable exception, the Cicadidae family, with bloated genomes (5,238 < 1C < 7,000 Mb) of a size previously known to exist only within Orthoptera. The genome sizes in all other Hemimetabolous orders (521 Mb < 1C < 3,860 Mb) fall within the range exhibited by the Hemiptera. New preparations and procedures for insect genome estimates with frozen material are described and discussed. To further improve and extend the number and quality of arthropod genome size estimates, a new flow cytometry insect standard, Blattaria: Periplaneta americana♂ (1C = 3,338 Mb) is established for genome size determination of insect genomes in excess of 2,000 Mb.

Eukaryotic genomes show tremendous size variation across taxa. Proximate explanations for genome size variation include differences in ploidy and amounts of noncoding DNA, especially repetitive DNA. Ultimate explanations include selection on physiological correlates of genome size such as cell size, which in turn influence body size, resulting in the often-observed correlation between body size and genome size. In this study, we examined body size and repetitive DNA elements in relationship to the evolution of genome size in North American representatives of a single beetle family, the Lampyridae (fireflies). The 23 species considered represent an excellent study system because of the greater than 5-fold range of genome sizes, documented here using flow cytometry, and the 3-fold range in body size, measured using pronotum width. We also identified common genomic repetitive elements using low-coverage sequencing. We found a positive relationship between genome size and repetitive DNA, particularly retrotransposons. Both genome size and these elements were evolving as expected given phylogenetic relatedness. We also tested whether genome size varied with body size and found no relationship. Together, our results suggest that genome size is evolving neutrally in fireflies.

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.

Drosophila melanogaster Meigen, 1830 has served as a model insect for over a century. Sequencing of the 11 additional Drosophila Fallen, 1823 species marks substantial progress in comparative genomics of this genus. By comparison, practically nothing is known about the genome size or genome sequences of parasitic wasps of Drosophila. Here, we present the first comparative analysis of genome size and karyotype structures of Drosophila parasitoids of the Leptopilina Förster, 1869 and Ganaspis Förster, 1869 species. The gametic genome size of Ganaspis xanthopoda (Ashmead, 1896) is larger than those of the three Leptopilina species studied. The genome sizes of all parasitic wasps studied here are also larger than those known for all Drosophila species. Surprisingly, genome sizes of these Drosophila parasitoids exceed the average value known for all previously studied Hymenoptera. The haploid chromosome number of both Leptopilina heterotoma (Thomson, 1862) and Leptopilina victoriae Nordlander, 1980 is ten. A chromosomal fusion appears to have produced a distinct karyotype for Leptopilina boulardi (Barbotin, Carton et Keiner-Pillault, 1979)(n = 9), whose genome size is smaller than that of wasps of the Leptopilina heterotoma clade. Like Leptopilina boulardi, the haploid chromosome number for Ganaspis xanthopoda is also nine. Our studies reveal a positive, but non linear, correlation between the genome size and total chromosome length in Drosophila parasitoids. These Drosophila parasitoids differ widely in their host range, and utilize different infection strategies to overcome host defense. Their comparative genomics, in relation to their exceptionally well-characterized hosts, will prove to be valuable for understanding the molecular basis of the host-parasite arms race and how such mechanisms shape the genetic structures of insectcommunities.