Molecular phylogeny of twelve Asian species of epilachnine ladybird beetles (Coleoptera, Coccinellidae) with notes on the direction of host shifts

Genetic divergence with ongoing gene flow is maintained by the use of different hosts in phytophagous ladybird beetles genus Henosepilachna

Adaptation to different environments can promote population divergence via natural selection even in the presence of gene flow - a phenomenon that typically occurs during ecological speciation. To elucidate how natural selection promotes and maintains population divergence during speciation, we investigated the population genetic structure, degree of gene flow and heterogeneous genomic divergence in three closely related Japanese phytophagous ladybird beetles: Henosepilachna pustulosa, H. niponica and H. yasutomii. These species act as a generalist, a wild thistle (Cirsium spp.) specialist and a blue cohosh (Caulophyllum robustum) specialist, respectively, and their ranges differ accordingly. The two specialist species widely co-occur but are reproductively isolated solely due to their high specialization to a particular host plant. Genomewide amplified fragment-length polymorphism (AFLP) markers and mitochondrial cytochrome c oxidase subunit I (COI) gene sequences demonstrated obvious genomewide divergence associated with both geographic distance and ecological divergence. However, a hybridization assessment for both AFLP loci and the mitochondrial sequences revealed a certain degree of unidirectional gene flow between the two sympatric specialist species. Principal coordinates analysis (PCoA) based on all of the variable AFLP loci demonstrated that there are genetic similarities between populations from adjacent localities irrespective of the species (i.e. host range). However, a further comparative genome scan identified a few fractions of loci representing approximately 1% of all loci as different host-associated outliers. These results suggest that these three species had a complex origin, which could be obscured by current gene flow, and that ecological divergence can be maintained with only a small fraction of the genome is related to different host use even when there is a certain degree of gene flow between sympatric species pairs.

Phylogeny of epilachna, henosepilachna, and some minor genera of phytophagous ladybird beetles (coleoptera: coccinellidae: coccinellinae: epilachnini), with an analysis of ancestral biogeography and host-plant utilization

Ladybird beetles in the tribe Epilachnini include notorious crop pests and model species studied intensively in various fields of evolutionary biology. From a combined dataset of mitochondrial (ND2) and nuclear (28S) DNA sequences, we reconstructed the phylogeny of 46 species of Epilachnini from Asia, Africa, America, and the Australian region: 16 species in Epilachna, 24 species in Henosepilachna, and one species each in Adira, Afidenta, Afidentula, Afissula, Chnootriba, and Epiverta. In our phylogenetic trees, both Epilachna and Henosepilachna were reciprocally polyphyletic. Asian Epilachna species were monophyletic, except for the inclusion of Afissula sp. Asian and Australian Henosepilachna species likewise formed a monophyletic group, excluding H. boisduvali. African Epilachna and Henosepilachna species did not group with their respective Asian and American congeners, but were paraphyletic to other clades (Epilachna species) or formed a separate monophyletic group (Henosepilachna species) together with Chnootriba similis. The American Epilachna species were monophyletic and formed a clade with American Adira clarkii and Asian Afidentula manderstjernae bielawskii; this clade was the sister group to Asian and Australian Henosepilachna, but was distant from Asian Epilachna. Chnootriba was embedded in the African Henosepilachna clade, and Afissula in the Asian Epilachna clade. Epiverta, which is morphologically unique, was the sister group to Asian Epilachna, although with weak support. From reconstructions of biogeographical distribution and host-plant utilization at ancestral nodes, we inferred an African origin for the common ancestor of the species studied, and found the frequency of host shifts to differ greatly between the two major lineages of Epilachnini examined.

Contribution of multiple isolating barriers to reproductive isolation between a pair of phytophagous ladybird beetles

Reproductive isolation between species may often be attained by multiple isolating barriers, but the components are rarely studied in animal taxa. To elucidate the nature of multiple isolating barriers, we quantified the strength of three premating barriers, including ecologically based ones (seasonal, habitat, and sexual), two postmating-prehatching barriers (reduced egg hatchability and conspecific sperm precedence [CSP]), and one posthatching barrier, including four components of F(1) hybrid reduced fitness, between two phytophagous ladybird beetles, Henosepilachna vigintioctomaculata and H. pustulosa. We detected five positive barriers (habitat isolation, sexual isolation, reduced egg hatchability, CSP, and reduced egg hatchability in backcrosses of F(1) hybrids). None of these barriers entirely prevents gene exchange when it acts alone, but jointly they generate nearly complete reproductive isolation even between sympatric populations. Host fidelity contributed most strongly to reproductive isolation by reducing interspecific hybridization through several important types of ecological isolation, including microspatial, habitat, and seasonal isolation. The existence of multiple isolating barriers likely helps keep reproductive isolation stable and robust, by complementing changes in the strength of leaky barriers. This complementarity of multiple isolating barriers yields the concept of robustness of reproductive isolation, which is important when considering the long-term maintenance of species boundaries in coexisting species pairs.

Differential gene expression between the embryonic tail bud and regenerating larval tail in Xenopus laevis

The regeneration of the amputated tail of Xenopus laevis larvae is an excellent model system for regeneration research. The wound left by the amputated tail is covered with epidermis within 24 h. Then, the cell number increases near the amputation plane at the notochord, spinal cord and muscle regions. An apparently complete tail with notochord, muscle and spinal cord is regenerated within two weeks. To reveal whether the molecular mechanism underlying the tail regeneration is the same as that in embryonic tail development, the gene expression patterns of the embryonic tail bud and the regenerating tail were compared by in situ hybridization and reverse transcription-polymerase chain reaction. Most genes analyzed were expressed at similar levels in both tissues, whereas two bone morphogenetic protein (BMP)-antagonists, chordin and noggin, were detected only in the embryonic tail bud. The regenerating tail also lacked expression of Xshh in the floor plate and expression of Xdelta-1 in the spinal cord and presomitic mesoderm. These results show that there are some differences in gene expression between the two processes. Furthermore, when the tail of Xenopus larvae is amputated, the regenerating tail has a gene expression pattern similar to the distal portion of the larval tail rather than the embryonic tail bud, suggesting that the cut larval tail does not make a new embryonic tail bud, but rather a new larval tail tip for regeneration.

Genetic architecture for normal and novel host-plant use in two local populations of the herbivorous ladybird beetle, Epilachna pustulosa

Trade-offs in host-plant use are thought to promote the evolution of host specificity. However, usually either positive or no genetic correlations have been found. Whereas factors enhancing variation in overall viability have been claimed to mask negative genetic correlations, alternative hypotheses emphasize the sequential changes in genetic correlation in the course of host-range evolution. In this study, the genetic architectures of performances on different hosts were compared in two populations of the herbivorous ladybird beetle, Epilachna pustulosa, using three host plants, one being normal for both, one novel for only one population, and the other novel for both populations. The genetic correlations between larval periods on normal hosts were significantly positive whereas those between normal and novel hosts were not different from zero. There was no evidence for reduced genetic variation on the normal host-plants. These results suggest that the host-range is not restricted by the antagonistic genetic associations among exploitation abilities on different plant species, but rather that selection of different host-plants may improve the coordination between genes responsible for the use of different plants.