Extensive Conserved Synteny of Genes between the Karyotypes of Manduca sexta and Bombyx mori Revealed by BAC-FISH Mapping

Abstract

BackgroundGenome sequencing projects have been completed for several species representing four highly diverged holometabolous insect orders, Diptera, Hymenoptera, Coleoptera, and Lepidoptera. The striking evolutionary diversity of insects argues a need for efficient methods to apply genome information from such models to genetically uncharacterized species. Constructing conserved synteny maps plays a crucial role in this task. Here, we demonstrate the use of fluorescence in situ hybridization with bacterial artificial chromosome probes as a powerful tool for physical mapping of genes and comparative genome analysis in Lepidoptera, which have numerous and morphologically uniform holokinetic chromosomes.Methodology/Principal FindingsWe isolated 214 clones containing 159 orthologs of well conserved single-copy genes of a sequenced lepidopteran model, the silkworm, Bombyx mori, from a BAC library of a sphingid with an unexplored genome, the tobacco hornworm, Manduca sexta. We then constructed a BAC-FISH karyotype identifying all 28 chromosomes of M. sexta by mapping 124 loci using the corresponding BAC clones. BAC probes from three M. sexta chromosomes also generated clear signals on the corresponding chromosomes of the convolvulus hawk moth, Agrius convolvuli, which belongs to the same subfamily, Sphinginae, as M. sexta.Conclusions/SignificanceComparison of the M. sexta BAC physical map with the linkage map and genome sequence of B. mori pointed to extensive conserved synteny including conserved gene order in most chromosomes. Only a few rearrangements, including three inversions, three translocations, and two fission/fusion events were estimated to have occurred after the divergence of Bombycidae and Sphingidae. These results add to accumulating evidence for the stability of lepidopteran genomes. Generating signals on A. convolvuli chromosomes using heterologous M. sexta probes demonstrated that BAC-FISH with orthologous sequences can be used for karyotyping a wide range of related and genetically uncharacterized species, significantly extending the ability to develop synteny maps for comparative and functional genomics.

Extensive synteny conservation of holocentric chromosomes in Lepidoptera despite high rates of local genome rearrangements

“…Overall conservation of synteny in Lepidoptera has been reported in several studies (16)(17)(18)(19)(20) suggesting that, over the 100 My separating the Bombyx and butterfly lineages, some degree of gene synteny has thus been maintained. However, a more detailed study was warranted to allow more general conclusions.…”

“…The remainder of the chromosomes showed one or more rearrangements or inversions. Similarly, 124 of 131 orthologues retained the same order on the chromosomes of B. mori and Manduca sexta (20), and these authors noted the paradox of conserved synteny on holocentric chromosomes, in which increased chromosome rearrangements are more likely. With the higher resolution of BAC sequencing, high colinearity of genes was observed between two related species of Heliconius, H. melpomene, and H. erato, in two regions covering 180 kb and 280 kb of sequence (18).…”

Step-by-step evolution of neo-sex chromosomes in geographical populations of wild silkmoths, Samia cynthia ssp.

“…Molecular linkage analyses revealed only a few orthologues of B. mori Z-linked genes that were also located on the Z chromosomes in other lepidopteran species, such as the European corn borer Ostrinia nubilalis (Dopman et al, 2005), Heliconius butterflies (Jiggins et al, 2005;Pringle et al, 2007) and the butterfly B. anynana ( Van't Hof et al, 2008;Beldade et al, 2009). Recently, BAC clones containing M. sexta orthologues of four B. mori Z-linked genes were mapped to the Z chromosomes of two species of the family Sphingidae, M. sexta and Agrius convolvuli, indicating the conserved synteny of the Z-linked genes between the three species (Yasukochi et al, 2009 Figure 5 Schematic drawing of step-by-step evolution of the sex chromosome constitution in female silkmoth of Samia cynthia subspecies, based on comparative mapping of orthologues of Bombyx mori genes. In two Japanese subspecies, the neo-sex chromosomes evolved in two steps by fusions of the ancestral sex chromosomes with autosomes.…”

“…Gene mapping in these species was carried out either by genetic linkage analysis (B. mori, H. melpomene and B. anynana) or by physical localization of genes using bacterial artificial chromosomes-fluorescence in situ hybridization (FISH with BAC of B. mori and M. sexta as probes) (Jiggins et al, 2005;Yoshido et al, 2005a;Yasukochi et al, 2006Yasukochi et al, , 2009Pringle et al, 2007;Beldade et al, 2009). Linkage analysis is a laborious and time-consuming procedure, especially in genetically uncharacterized species, and BAC-FISH mapping requires a BAC library, which is not available in S. cynthia.…”

Linkage map of the peppered moth, Biston betularia (Lepidoptera, Geometridae): a model of industrial melanism

“…Lepidoptera are an interesting group in this respect, characterized by wide variation in chromosome number (Suomalainen and Brown, 1984;Kandul et al, 2007) around a strong, putatively ancestral mode (haploid n ¼ 31), encountered in approximately one-third of the species (White, 1973). The dynamism implied by high rates of chromosome fission/fusion contrasts with a highly conserved synteny of genes between chromosomes across distantly related taxa (Pringle et al, 2007;Sahara et al, 2007;Beldade et al, 2009;Yasukochi et al, 2009;d'Alenc¸on et al, 2010;Baxter et al, 2011). This latter feature allows unambiguous identification of orthologous chromosomal regions in different species to reconstruct the events that produced the current pattern of structural diversity.…”