Shun-Chern Tsaur

Cryptic species are often defined as species that cannot be reliably distinguished from some known species due to their morphological similarity. Such species are casually discovered from collections of known species. In the present study, we report the discovery of three cryptic species of the subgenus Sophophora (genus Drosophila) belonging to the montium and melanogaster species groups from laboratory-maintained stocks. Based on our morphological examination, cross experiments, and molecular phylogenetic analyses, the three species were recognized as new to science and described as D. parabicornuta sp. nov., D. xitouensis sp. nov., and D. neolutea sp. nov.

A total of 58 (eight known and 50 new) species of the subgenus Stegana (Steganina) from China were surveyed and (re)described: S. (S.) bacilla Chen & Aotsuka, 2004, S. (S.) belokobylskiji Sidorenko, 1997, S. (S.) hirticeps Wang, Gao, & Chen, 2013, S. (S.) izu Sidorenko, 1997, S. (S.) kanmiyai Okada & Sidorenko, 1992, S. (S.) masanoritodai Okada & Sidorenko, 1992, S. (S.) maymyo Sidorenko, 1997, stat. rev., S. (S.) nigripes Zhang & Chen, 2015, S. (S.) alafoliacea Zhang & Chen, sp. nov., S. (S.) baoxing Li & Chen, sp. nov., S. (S.) bibarbata Li & Chen, sp. nov., S. (S.) bimai Cui & Chen, sp. nov., S. (S.) cinereipecta Zhang & Chen, sp. nov., S. (S.) cardua Cui & Chen, sp. nov., S. (S.) cordhirsuta Wang & Chen, sp. nov., S. (S.) cornuta Li & Chen, sp. nov., S. (S.) cucullata Li & Chen, sp. nov., S. (S.) cultella Cui & Chen, sp. nov., S. (S.) curvitabulata Cui & Chen, sp. nov., S. (S.) daiya Cui & Chen, sp. nov., S. (S.) dendrophila Zhang & Chen, sp. nov., S. (S.) flabella Li & Chen, sp. nov., S. (S.) flavipes Li & Chen, sp. nov., S. (S.) formosa Zhang & Chen, sp. nov., S. (S.) fusca Li & Chen, sp. nov., S. (S.) fuscipes Li & Chen, sp. nov., S. (S.) glaucopalpula Cui & Chen, sp. nov., S. (S.) haba Zhang & Chen, sp. nov., S. (S.) hirticlavata Cui & Chen, sp. nov., S. (S.) iaspidea Zhang & Chen, sp. nov., S. (S.) idiasta Cui & Chen, sp. nov., S. (S.) kanda Cui & Chen, sp. nov., S. (S.) labao Li & Chen, sp. nov., S. (S.) lancang Li & Chen, sp. nov., S. (S.) latifoliacea Wang & Chen, sp. nov., S. (S.) liusanjieae Li & Chen, sp. nov., S. (S.) magniflava Cui & Chen, sp. nov., S. (S.) mailangang Li & Chen, sp. nov., S. (S.) marenubila Cui & Chen, sp. nov., S. (S.) menghai Zhang & Chen, sp. nov., S. (S.) menglian Li & Chen, sp. nov., S. (S.) minutiflava Li & Chen, sp. nov., S. (S.) multiprocera Li & Chen, sp. nov., S. (S.) nayun Li & Chen, sp. nov., S. (S.) nigridentata Wang & Chen, sp. nov., S. (S.) nigripalpula Cui & Chen, sp. nov., S. (S.) otphylla Cui & Chen, sp. nov., S. (S.) radiciflava Zhang & Chen, sp. nov., S. (S.) rava Cui & Chen, sp. nov., S. (S.) sciophila Li & Chen, sp. nov., S. (S.) septencolorata Li & Chen, sp. nov., S. (S.) serrata Zhang & Chen, sp. nov., S. (S.) silvestrella Zhang & Chen, sp. nov., S. (S.) simola Cui & Chen, sp. nov., S. (S.) yani Li & Chen, sp. nov., S. (S.) yixiang Zhang & Chen, sp. nov., S. (S.) zaduo Cui & Chen, sp. nov., and S. (S.) zhuoma Cui & Chen, sp. nov. We also provided a complete list of Chinese Steganina species together with their geographical distributions. In addition, the majority of currently available DNA barcode (partial sequence of the mitochondrial cytochrome c oxidase subunit I (COI) gene) sequences of this subgenus (435 sequences of 102 spp.) were employed in a molecular analysis for species delimitation. Taken together, morphology- and molecular-based species delimitation results reached a consensus for an overwhelming majority of these Steganina species (98 of 102 spp.).

Gut bacteria may play roles in the dietary utilization of hosts, especially in specialist animals, during long-term host-microbe interaction. By comparing the gut bacterial communities between specialist and generalist drosophilid flies, we found that specialists harbor more bacteria linked to complex carbohydrate degradation, amino acid metabolism, vitamin B ₁₂ formation, and detoxification than do generalists.

Widespread premature termination codon mutations (PTCs) were recently observed in human and fly populations. We took advantage of the population resequencing data in the Drosophila Genetic Reference Panel to investigate how the expression profile and the evolutionary age of genes shaped the allele frequency distribution of PTCs. After generating a high-quality data set of PTCs, we clustered genes harboring PTCs into three categories: genes encoding low-frequency PTCs (≤1.5%), moderate-frequency PTCs (1.5–10%), and high-frequency PTCs (>10%). All three groups show narrow transcription compared with PTC-free genes, with the moderate- and high-PTC frequency groups showing a pronounced pattern. Moreover, nearly half (42%) of the PTC-encoding genes are not expressed in any tissue. Interestingly, the moderate-frequency PTC group is strongly enriched for genes expressed in midgut, whereas genes harboring high-frequency PTCs tend to have sex-specific expression. We further find that although young genes born in the last 60 My compose a mere 9% of the genome, they represent 16%, 30%, and 50% of the genes containing low-, moderate-, and high-frequency PTCs, respectively. Among DNA-based and RNA-based duplicated genes, the child copy is approximately twice as likely to contain PTCs as the parent copy, whereas young de novo genes are as likely to encode PTCs as DNA-based duplicated new genes. Based on these results, we conclude that expression profile and gene age jointly shaped the landscape of PTC-mediated gene loss. Therefore, we propose that new genes may need a long time to become stably maintained after the origination.

Frequent gene duplications in the genome incessantly supply new genetic materials for functional innovation presumably driven by positive Darwinian selection. This mechanism in the desaturase gene family has been proposed to be important in triggering the pheromonal diversification in insects. With the recent completion of a dozen Drosophila genomes, a genome-wide perspective is possible. In this study, we first identified homologs of desaturase genes in 12 Drosophila species and noted that while gene duplication events are relatively frequent, gene losses are not scarce, especially in the desat1–desat2–desatF clade. By reconciling the gene tree with species phylogeny and the chromosomal synteny of the sequenced Drosophila genomes, at least one gene loss in desat2 and a minimum of six gene gains (resulting in seven desatF homologs, α-η), three gene losses and one relocation in desatF were inferred. Upon branching off the ancestral desat1 lineage, both desat2 and desatF gained novel functions through accelerating protein evolution. The amino acid residues under positive selection located near the catalytic sites and the C-terminal region might be responsible for altered substrate selectivity between closely related species. The association between the expression pattern of desatF-α and the chemical composition of cuticular hydrocarbons implies that the ancestral function of desatF-α is the second desaturation at the four carbons after the first double bond in diene synthesis, and the shift from bisexual to female-specific expression in desatF-α occurred in the ancestral lineage of Drosophila melanogaster subgroup. A relationship between the number of expressed desatF homologs and the diene diversification has also been observed. These results suggest that the molecular diversification of fatty acid desaturases after recurrent gene duplication plays an important role in pheromonal diversity in Drosophila.

The importance of gene duplication in evolution has long been recognized. Because duplicated genes are prone to diverge in function, gene duplication could plausibly play a role in species differentiation. However, experimental evidence linking gene duplication with speciation is scarce. Here, we show that a hybrid-male sterility gene, Odysseus ( OdsH ), arose by gene duplication in the Drosophila genome. OdsH has evolved at a very high rate, whereas its most immediate paralog, unc-4 , is nearly identical among species in the Drosophila melanogaster subgroup. The disparity in their sequence evolution is echoed by the divergence in their expression patterns in both soma and reproductive tissues. We suggest that duplicated genes that have yet to evolve a stable function at the time of speciation may be candidates for “speciation genes,” which is broadly defined as genes that contribute to differential adaptation between species.

The cuticular hydrocarbon (CH) pheromones in Drosophila melanogaster exhibit strong geographic variation. African and Caribbean populations have a high ratio of 5,9 heptacosadiene/7,11 heptacosadiene (the “High” CH type), whereas populations from all other areas have a low ratio (“Low” CH type). Based on previous genetic mapping, DNA markers were developed that localized the genetic basis of this CH polymorphism to within a 13-kb region. We then carried out a hierarchical search for diagnostic nucleotide sites starting with four lines, and increasing to 24 and 43 lines from a worldwide collection. Within the 13-kb region, only one variable site shows a complete concordance with the CH phenotype. This is a 16-bp deletion in the 5′ region of a desaturase gene ( desat2 ) that was recently suggested to be responsible for the CH polymorphism on the basis of its expression [Dallerac, R., Labeur, C., Jallon, J.-M., Knipple, D. C., Roelofs, W. L. & Wicker-Thomas, C. (2000) Proc. Natl. Acad. Sci. 97, 9449–9454]. The cosmopolitan Low type is derived from the ancestral High type, and DNA sequence variations suggest that the former spread worldwide with the aid of positive selection. Whether this CH variation could be a component of the sexual isolation between Zimbabwe and other cosmopolitan populations remains an interesting and unresolved question.