Recency, range expansion, and unsorted lineages: implications for interpreting neutral genetic variation in the sharp-tailed grouse (Tympanuchus phasianellus).

Both current and historical patterns of variation are relevant to understanding and managing ecological diversity. Recently derived species present a challenge to the reconstruction of historical patterns because neutral molecular data for these taxa are more likely to exhibit effects of recent and ongoing demographic processes. We studied geographical patterns of neutral molecular variation in a species thought to be of relatively recent origin, Tympanuchus phasianellus (sharp-tailed grouse), using mitochondrial control region sequences (CR-I), amplified fragment length polymorphisms (AFLP), and microsatellites. For historical context, we also analysed CR-I in all species of Tympanuchus. Within T. phasianellus, we found evidence for restricted gene flow between eastern and western portions of the species range, generally corresponding with the range boundary of T. p. columbianus and T. p. jamesi. The mismatch distribution and molecular clock estimates from the CR-I data suggested that all Tympanuchus underwent a range expansion prior to sorting of mitotypes among the species, and that sorting may have been delayed as a result of mutation-drift disequilibrium. This study illustrates the challenge of using genetic data to detect historical divergence in groups that are of relatively recent origin, or that have a history dominated by nonequilibrium conditions. We suggest that in such cases, morphological, ecological, and behavioural data may be particularly important adjuncts to molecular data for the recognition of historically or adaptively divergent groups.

Psyllid endosymbionts exhibit patterns of co-speciation with hosts and destabilizing substitutions in ribosomal RNA.

Eubacterial 16S rDNAs were sequenced from endosymbionts of seven psyllids (Psylloidea) and one whitefly (Aleyrodoidea), to investigate the evolution of endosymbionts and their hosts. Primary endosymbionts from all psyllids formed a highly supported clade, tentatively placed as the sister to whitefly primary endosymbionts, and showing several points of congruence with the host morphological phylogeny. Almost all host taxa yielded an additional eubacterial sequence, related either to known psyllid secondary endosymbionts or to other insect endosymbionts or parasites. The relationships of some secondary endosymbionts also suggested cospeciation with psyllid hosts, or ancient horizontal transfers. All primary endosymbionts, and some secondary endosymbionts, exhibited molecular genetic effects of a long-term, intracellular existence in their biased nucleotide content and decreased stability of rRNA secondary structure.

Phylogenetic characterization and molecular evolution of bacterial endosymbionts in psyllids (Hemiptera: Sternorrhyncha).

Most sternorrhynchan insects harbor endosymbiotic bacteria in specialized cells (bacteriocytes) near the gut which provide essential nutrients for hosts. In lineages investigated so far with molecular methods (aphids, mealybugs, whiteflies), endosymbionts apparently have arisen from independent infections of common host ancestors and co-speciated with their hosts. Some endosymbionts also exhibit putatively negative genetic effects from their symbiotic association. In this study, the identity of endosymbionts in one major sternorrhynchan lineage, psyllids (Psylloidea), was investigated to determine their position in eubacterial phylogeny and their relationship to other sternorrhynchan endosymbionts. Small-subunit ribosomal RNA genes (16S rDNA) from bacteria in three psyllid species (families Psyllidae and Triozidae) were sequenced and incorporated into an alignment including other insect endosymbionts and free-living bacteria. In phylogenetic analysis, all sequences were placed within the gamma subdivision of the Proteobacteria. Three sequences, one from each psyllid species, formed a highly supported monophyletic group whose branching order matched the host phylogeny, and also exhibited accelerated rates of evolution and mutational bias toward A and T nucleotides. These attributes, characteristic of primary (P) bacteriocyte-dwelling endosymbionts, suggested that these sequences were from the putative psyllid P endosymbiont. Two other sequences were placed within the gamma-3 subgroup of Proteobacteria and were hypothesized to be secondary endosymbionts. The analysis also suggested a sister relationship between P endosymbionts of psyllids and whiteflies. Thus, a continuous mutualistic association between bacteria and insects may have existed since the common ancestor of psyllids and whiteflies. Calculations using a universal substitution rate in bacteria corrected for endosymbiont rate acceleration support the idea that this common ancestor was also the ancestor of all Sternorrhyncha. Compared with other P endosymbiont lineages, the genetic consequences of intracellular life for some psyllid endosymbionts have been exaggerated, indicating possible differences in population structures of bacteria and/or hosts.