Lucinda P. Lawson

STRONG SELECTION AGAINST HYBRIDS AT A HYBRID ZONE IN THE ENSATINA RING SPECIES COMPLEX AND ITS EVOLUTIONARY IMPLICATIONS

Abstract The analysis of interactions between lineages at varying levels of genetic divergence can provide into the process of speiation through the accumulation of incompatiable mutations. Ring species, and especially the Ensatina eschscholtzii system exemplify this approach. The plethodontid salamanders E. eschscholtzii xanthoptica and E. eschscholtzii platensis hybridize in the central Sierran foothills of California. We compared the genetic structure across two transects (southern and northern Calaveras Co.), one of which was resampled over 20 years, and examined diagnostic molecular markers (eight allozyme loci and mitochondrial DNA) and a diagnostic quantitative trait (color pattern). Key results across all studies were: (1) cline centers for all markers were coincident and the zones were narrow, with width estimates of 730 m to 2000 m; (2) cline centers at the northern Calveras transect were coincident between 1981 and 2001, demonstrating repeatability over five generations; (3) there were very few if any putative F1S, but a relatively high number of backcrossed individuals in the central portion of transects; and (4) we found substantial linkage disequilibrium in all three studies and strong heterozygote deficit both in northern Calaveras, in 2001, and southern Calaveras. Both linkage disequilibrium and heterozygote deficit showed maximum values near the center of the zones. Using estimates of cline width and dispersal, we infer strong selection against hybrids. This is sufficient to promote accumulation of differences at loci that are neutral or under divergent selection, but would still allow for introgression of adaptive alleles. The evidence for strong but incomplete isolation across this centrally located contact is consistent with theory suggesting a gradual increase in postzygotic incompatibility between allopatric populations subject to divergent selection and reinforces the value of Ensatina as a system for the study of divergence and speciation at multiple stages.

The discordance of diversification: evolution in the tropical-montane frogs of the Eastern Arc Mountains of Tanzania

Species with similar geographical distribution patterns are often assumed to have a shared biogeographical history, an assumption that can be tested with a combination of molecular, spatial, and environmental data. This study investigates three lineages of Hyperolius frogs with concordant ranges within the Eastern Afromontane Biodiversity Hotspot to determine whether allopatric populations of co‐distributed lineages shared a parallel biogeographical response to their shared paleoclimatic histories. The roles of refugial distributions, isolation, and climate cycles in shaping their histories are examined through Hierarchical Approximate Bayesian Computation, comparative phylogeography, and comparisons of current and past geographical distributions using ecological niche models. Results from these analyses show these three lineages to have independent evolutionary histories, which current spatial configurations of sparsely available habitat (montane wetlands) have moulded into convergent geographical ranges. In spite of independent phylogeographical histories, diversification events are temporally concentrated, implying that past vicariant events were significant at the generic level. This mixture of apparently disparate histories is likely due to quantifiably different patterns of expansion and retreat among species in response to past climate cycles. Combining climate modelling and phylogeographical data can reveal unrecognized complexities in the evolution of co‐distributed taxa.

THE EVOLUTIONARY HISTORY OF DARWIN'S FINCHES: SPECIATION, GENE FLOW, AND INTROGRESSION IN A FRAGMENTED LANDSCAPE

Many classic examples of adaptive radiations take place within fragmented systems such as islands or mountains, but the roles of mosaic landscapes and variable gene flow in facilitating species diversification is poorly understood. Here we combine phylogenetic and landscape genetic approaches to understand diversification in Darwins finches, a model adaptive radiation. We combined sequence data from 14 nuclear introns, mitochondrial markers, and microsatellite variation from 51 populations of all 15 recognized species. Phylogenetic species‐trees recovered seven major finch clades: ground, tree, vegetarian, Cocos Island, grey and green warbler finches, and a distinct clade of sharp‐beaked ground finches (Geospiza cf. difficilis) basal to all ground and tree finches. The ground and tree finch clades lack species‐level phylogenetic structure. Interisland gene flow and interspecies introgression vary geographically in predictable ways. First, several species exhibit concordant patterns of population divergence across the channel separating the Galápagos platform islands from the separate volcanic province of northern islands. Second, peripheral islands have more admixed populations while central islands maintain more distinct species boundaries. This landscape perspective highlights a likely role for isolation of peripheral populations in initial divergence, and demonstrates that peripheral populations may maintain genetic diversity through outbreeding during the initial stages of speciation.

Diversification in a biodiversity hot spot: landscape correlates of phylogeographic patterns in the African spotted reed frog.

The Eastern Afromontane Biodiversity Hotspot is known for microendemism and exceptional population genetic structure. The regions landscape heterogeneity is thought to limit gene flow between fragmented populations and create opportunities for regional adaptation, but the processes involved are poorly understood. Using a combination of phylogeographic analyses and circuit theory, I investigate how characteristics of landscape heterogeneity including regional distributions of slope, rivers and streams, habitat and hydrological basins (drainages) impact genetic distance among populations of the endemic spotted reed frog (Hyperolius substriatus), identifying corridors of connectivity as well as barriers to dispersal. Results show that genetic distance among populations is most strongly correlated to regional and local hydrologic structure and the distribution of suitable habitat corridors, not isolation by distance. Contrary to expectations, phylogeographic structure is not coincident with the two montane systems, but instead corresponds to the split between the regions two major hydrological basins (Zambezi and East Central Coastal). This results in a paraphyletic relationship for the Malawian Highlands populations with respect to the Eastern Arc Mountains and implies that the northern Malawian Highlands are the diversity centre for H. substriatus. Although the Malawian Highlands collectively hold the greatest genetic diversity, individual populations have lower diversity than their Eastern Arc counterparts, with an overall pattern of decreasing population diversity from north to south. Through the study of intraspecific differentiation across a mosaic of ecosystem and geographic heterogeneity, we gain insight into the processes of diversification and a broader understanding of the role of landscape in evolution.

Male reproductive fitness and queen polyandry are linked to variation in the supergene Gp-9 in the fire ant Solenopsis invicta

Supergenes are clusters of tightly linked loci maintained in specific allelic combinations to facilitate co-segregation of genes governing adaptive phenotypes. In species where strong selection potentially operates at different levels (e.g. eusocial Hymenoptera), positive selection acting within a population to maintain specific allelic combinations in supergenes may have unexpected consequences for some individuals, including the preservation of disadvantageous traits. The nuclear gene Gp-9 in the invasive fire ant Solenopsis invicta is part of a non-recombining, polymorphic supergene region associated with polymorphism in social organization as well as traits affecting physiology, fecundity and behaviour. We show that both male reproductive success and facultative polyandry in queens have a simple genetic basis and are dependent on male Gp-9 genotype. Gp-9b males are unable to maintain exclusive reproductive control over their mates such that queens mated to Gp-9b males remain highly receptive to remating. Queens mated to multiple Gp-9B males are rare. This difference appears to be independent of mating plug production in fertile males of each Gp-9 genotype. However, Gp-9b males have significantly lower sperm counts than Gp-9B males, which could be a cue to females to seek additional mates. Despite the reduced fitness of Gp-9b males, polygyne worker-induced selective mortality of sexuals lacking b-like alleles coupled with the overall success of the polygyne social form act to maintain the Gp-9b allele within nature. Our findings highlight how strong worker-induced selection acting to maintain the Gp-9b allele in the polygyne social form may simultaneously result in reduced reproductive fitness for individual sexual offspring.

Herpetofauna of Montane Areas of Tanzania. 3. Amphibian Diversity in the Northwestern Eastern Arc Mountains, with the Description of a New Species of Arthroleptis (Anura: Arthroleptidae)

Abstract Amphibians of the Eastern Arc Mountains are poorly known, apart from those of the East Usambara, Uluguru, and Udzungwa Mountains. Here, we detail specimens collected in the northwestern Eastern Arc Mountains (West Usambara, South Pare, and North Pare) over the past 100 years. Although not comprehensive, the list is compiled from specimens held at institutions in Europe and the United States and in personal collections; it includes most of the specimens known from this area. From these specimens, we describe one new species of the frog genus Arthroleptis and discuss other specimens that may represent new species. We add substantially to the known diversity in these areas. Using the compiled species list, we assess species diversity and relate the differences to sampling bias and possible biogeographic differences among the mountains. The future preservation of amphibians across this area is of concern given the increasing loss of forest.

Three new species of spiny throated reed frogs (Anura: Hyperoliidae) from evergreen forests of Tanzania

BackgroundThe East African spiny-throated reed frog complex (Hyperolius spinigularis, H. tanneri, and H. minutissimus) is comprised of morphologically similar species with highly fragmented populations across the Eastern Afromontane Region. Recent genetic evidence has supported the distinctiveness of populations suggesting a number of cryptic species. We analyse newly collected morphological data and evaluate the taxonomic distinctiveness of populations.ResultsWe find three new distinct species on the basis of morphological and molecular evidence. The primary morphological traits distinguishing species within the Hyperolius spinigularis complex include the proportions and degree of spinosity of the gular flap in males and snout-urostyle length in females. Other features allow the three species to be distinguished from each other (genetics). We refine the understanding of H. minutissimus which can be found in both forest and grassland habitats of the Udzungwa Mountains, and provide more details on the call of this species. Further details on ecology are noted for all species where known.ConclusionsThree new species are described and we narrow the definition and distribution of Hyperolius spinigularis and H. minutissimus in East Africa. The spiny-throated reed frogs have highly restricted distributions across the fragmented mountains of the Eastern Afromontane region. Given the newly defined and substantially narrower distributions of these spiny-throated reed frog species, conservation concerns are outlined.

Evolutionary history of the grey-faced Sengi, Rhynchocyon udzungwensis, from Tanzania: a molecular and species distribution modelling approach.

Rhynchocyon udzungwensis is a recently described and poorly understood sengi (giant elephant-shrew) endemic to two small montane forests in Southern Tanzania, and surrounded in lower forests by R. cirnei reichardi. In this study, we investigate the molecular genetic relationship between R. udzungwensis and R. c. reichardi, and the possible role that shifting species distributions in response to climate fluctuations may have played in shaping their evolutionary history. Rhynchocyon udzungwensis and R. c. reichardi individuals were sampled from five localities for genetic analyses. Three mitochondrial and two nuclear loci were used to construct species trees for delimitation and to determine whether introgression was detectable either from ancient or ongoing hybridization. All species-tree results show R. udzungwensis and R. c. reichardi as distinct lineages, though mtDNA shows evidence of introgression in some populations. Nuclear loci of each species were monophyletic, implying introgression is exclusively historical. Because we found evidence of introgression, we used distribution data and species distribution modelling for present, glacial, and interglacial climate cycles to predict how shifting species distributions may have facilitated hybridization in some populations. Though interpretations are affected by the limited range of these species, a likely scenario is that the mtDNA introgression found in eastern mid-elevation populations was facilitated by low numbers of R. udzungwensis that expanded into lowland heavily occupied R. c. reichardi areas during interglacial climate cycles. These results imply that relationships within the genus Rhynchocyon may be confounded by porous species boundaries and introgression, even if species are not currently sympatric.

Divergence at the edges: peripatric isolation in the montane spiny throated reed frog complex

BackgroundPeripatric speciation and peripheral isolation have uncertain importance in species accumulation, and are largely overshadowed by assumed dominance of allopatric modes of speciation. Understanding the role of different speciation mechanisms within biodiversity hotspots is central to understanding the generation of biological diversity. Here, we use a phylogeographic analysis of the spiny-throated reed frogs and examine sister pairings with unbalanced current distributional ranges for characteristics of peripatric speciation. We further investigate whether forest/grassland mosaic adapted species are more likely created through peripatric speciation due to instability of this habitat type.ResultsWe reconstructed a multi-locus molecular phylogeny of spiny-throated reed frogs which we then combined with comparative morphologic data to delimit species and analyze historical demographic change; identifying three new species. Three potential peripatric speciation events were identified along with one case of allopatric speciation. Peripatric speciation is supported through uneven potential and realized distributions and uneven population size estimates based on field collections. An associated climate shift was observed in most potentially peripatric splits. Morphological variation was highest in sexually dimorphic traits such as body size and gular shape, but this variation was not limited to peripatric species pairs as hypothesized. The potentially allopatric species pair showed no niche shifts and equivalent effective population sizes, ruling out peripatry in that speciation event. Two major ecological niche shifts were recovered within this radiation, possibly as adaptations to occupy areas of grassland that became more prevalent in the last 5 million years. Restricted and fluctuating grassland mosaics within forests might promote peripatric speciation in the Eastern Arc Biodiversity Hotspot (EABH).ConclusionsIn our case study, peripatric speciation appears to be an important driver of diversity within the EABH biodiversity hotspot, implying it could be a significant speciation mechanism in highly fragmented ecosystems. Extensive peripatric speciation in this montane archipelago may explain the abundance of discrete lineages within the limited area of the EABH, as inferred in remote island archipelagos. Future phylogenetic studies incorporating demographic and spatial analyses will clarify the role of peripatric speciation in creating biodiversity hotspots.

Impact of species delimitation and sampling on niche models and phylogeographical inference: A case study of the East African reed frog Hyperolius substriatus Ahl, 1931

Ecological niche models (ENMs) have been used in a wide range of ecological and evolutionary studies. In biogeographic studies these models have, among other things, helped in the discovery of new allopatric populations, and even new species. However, small sample sizes and questionable taxonomic delimitation can challenge models, often decreasing their accuracy. Herein we examine the sensitivity of ENMs to the addition of new, geographically isolated populations, and the impact of applying different taxonomic delimitations. The East African reed frog Hyperolius substriatus Ahl, 1931 was selected as a case study because it has been the subject of previous ENM predictions. Our results suggest that addition of new data and reanalysis of species lineages of H. substriatus improved our understanding of the evolutionary history of this group of frogs. ENMs provided robust predictions, even when some populations were deliberately excluded from the models. Splitting the lineages based on genetic relationships and analysing the ENMs separately provided insights about the biogeographical processes that led to the current distribution of H. substriatus.