Genetic diversity and gene flow in fragmented populations of the rare shrub, Calothamnus sp. Whicher

Ca/othamnus sp. Whicher (Myrtaceae) is a narrow range endemic shrub restricted to ironstone soils near the town of Busselton in south-western Australia. Due to extensive land clearing for agriculture and mineral exploration, the species is fragmented over the majority of its range. In the present study, microsatellite markers were used to characterise levels of genetic diversity and describe levels of differentiation and gene flow among seven small, isolated road verge populations. Allelic diversity within the taxon over the six microsatellites was high (A = 17.6 ± 1.6). Diversity within populations was considerably lower (A = 5.19 ± 1.27), and was positively correlated with population size. An excess of homozygotes and high fixation indices in all populations (mean FIS = 0.315 ± 0.13) indicated that inbreeding within populations was high. Estimates of the divergence in allele frequencies between populations (global θ = 0.256) and genetic distance (mean Nei\u27s D = 0.370) revealed a distinct genetic structure within the study sample. Direct estimates of gene flow, determined by assigning paternity to seed crops from the two largest populations, were low (2.7% and 4%), yet similar to historical estimates derived from the degree of differentiation between populations. However, due to the degree of inbreeding within these populations and their susceptibility to genetic drift, these historical estimates appeared to be a consequence of post fragmentation rates of gene flow rather than reflecting pre-fragmentation rates. Low levels of gene flow into the two largest populations and restricted within population patterns of mating were supported by high global (among population) and mean pairwise (within population) estimates of Φft, which represents the degree of differentiation between maternally sampled pollen pools. The differentiation observed between populations is most likely a result of postfragmentation processes rather than being driven by mutation and maintained by low levels of historical gene flow. The six natural road verge populations observed in this study were likely to be part of one or more larger, continuous populations similar to those which are located in relatively undisturbed fragments of natural vegetation. Initially, differentiation within these populations probably resulted from their small sizes and the heterogeneous fine scale genetic structure within the larger population(s) from which they originated. Further differentiation appears to have resulted from· ·extensive inbreeding within populations and the increased vulnerability to drift associated with decreasing population size. Results from other studies, including that conducted on the closely related species Calothamnus quadrifidus (Byrne et al., in press), suggest that fragmentation has reduced rates of gene flow from higher historical levels. However, the detection of some gene flow events across the breadth of the study site suggests that isolation itself was not preventing gene flow. Rather, the loss of natural vegetation may have reduced the abundance of bird pollinators. The conservation and evolutionary implications of these findings are discussed

Speciation in marine environments: Diving under the surface

Marine environments are inhabited by a broad representation of the tree of life, yet our understanding of speciation in marine ecosystems is extremely limited compared with terrestrial and freshwater environments. Developing a more comprehensive picture of speciation in marine environments requires that we 'dive under the surface' by studying a wider range of taxa and ecosystems is necessary for a more comprehensive picture of speciation. Although studying marine evolutionary processes is often challenging, recent technological advances in different fields, from maritime engineering to genomics, are making it increasingly possible to study speciation of marine life forms across diverse ecosystems and taxa. Motivated by recent research in the field, including the 14 contributions in this issue, we highlight and discuss six axes of research that we think will deepen our understanding of speciation in the marine realm: (a) study a broader range of marine environments and organisms; (b) identify the reproductive barriers driving speciation between marine taxa; (c) understand the role of different genomic architectures underlying reproductive isolation; (d) infer the evolutionary history of divergence using model‐based approaches; (e) study patterns of hybridization and introgression between marine taxa; and (f) implement highly interdisciplinary, collaborative research programmes. In outlining these goals, we hope to inspire researchers to continue filling this critical knowledge gap surrounding the origins of marine biodiversity

The evolution of strong reproductive isolation between sympatric intertidal snails

The evolution of strong reproductive isolation (RI) is fundamental to the origins and maintenance of biological diversity, especially in situations where geographical distributions of taxa broadly overlap. But what is the history behind strong barriers currently acting in sympatry? Using whole-genome sequencing and single nucleotide polymorphism genotyping, we inferred (i) the evolutionary relationships, (ii) the strength of RI, and (iii) the demographic history of divergence between two broadly sympatric taxa of intertidal snail. Despite being cryptic, based on external morphology, Littorina arcana and Littorina saxatilis differ in their mode of female reproduction (egg-laying versus brooding), which may generate a strong post-zygotic barrier. We show that egg-laying and brooding snails are closely related, but genetically distinct. Genotyping of 3092 snails from three locations failed to recover any recent hybrid or backcrossed individuals, confirming that RI is strong. There was, however, evidence for a very low level of asymmetrical introgression, suggesting that isolation remains incomplete. The presence of strong, asymmetrical RI was further supported by demographic analysis of these populations. Although the taxa are currently broadly sympatric, demographic modelling suggests that they initially diverged during a short period of geographical separation involving very low gene flow. Our study suggests that some geographical separation may kick-start the evolution of strong RI, facilitating subsequent coexistence of taxa in sympatry. The strength of RI needed to achieve sympatry and the subsequent effect of sympatry on RI remain open questions. This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'

Data from: Ecological speciation in an island snail: evidence for the parallel evolution of a novel ecotype and maintenance by ecologically dependent postzygotic isolation

Speciation is the process by which reproductive isolation evolves between populations. Two general models of speciation have been proposed: ecological speciation, where reproductive barriers evolve due to ecologically based divergent selection, and mutation-order speciation, where populations fix different mutations as they adapt to similar selection pressures. I evaluate these alternative models and determine the progress of speciation in a diverse group of land snails, genus Rhagada, inhabiting Rosemary Island. A recently derived keeled-flat morphotype occupies two isolated rocky hills, while globose-shelled snails inhabit the surrounding plains. The study of one hill reveals that they are separated by a narrow hybrid zone. As predicted by ecological speciation theory, there are local and landscape level associations between shell shape and habitat, and the morphological transition coincides with a narrow ecotone between the two distinct environments. Microsatellite DNA revealed a cline of hybrid index scores much wider than the morphological cline, further supporting the ecological maintenance of the morphotypes. The hybrid zone does not run through an area of low population density, as is expected for mutation-order hybrid zones, and there is a unimodal distribution of phenotypes at the centre, suggesting that there is little or no prezygotic isolation. Instead, these data suggest that the ecotypes are maintained by ecologically dependent postzygotic isolation (i.e. ecological selection against hybrids). Mitochondrial and Microsatellite DNA indicate that the keeled-flat form evolved recently, and without major historical disruptions to gene flow. The data also suggest that the two keeled-flat populations, inhabiting similar rocky hills, have evolved in parallel. These snails provide a complex example of ecological speciation in its early stages

Rosemary Rhagada COI sequences

666 bp fragments of the mitochondrial gene Cytochrome Oxidase subunit I from 263 snails collected from the main study area. The first number in the title is the transect number, the second is the site number and the third is the individual number (Transect_Site_Individual). The dataset is edited and aligned. Sequences with the same name followed by the letter B were collected from the same site on different occasions (i.e., 8_1_4 and 8_1_4B). The sequences have been edited and aligned

Data and code for: The genetic architecture of a recent transition to live-bearing in marine snails

This repository contains the code and VCF files needed to conduct the analyses in our MS. Each folder contains a readMe document explaining the nature of each file and dataset and the results and analyses that they relate to. The same anlaysis code (but not VCF files) is also available at https://github.com/seanstankowski/Littorina_reproductive_mod

Digest: On the origin of a possible hybrid species

Hybrid speciation—the origin of new species resulting from the hybridization of genetically divergent lineages—was once considered rare, but genomic data suggest that it may occur more often than once thought. In this study, Noguerales and Ortego found genomic evidence supporting the hybrid origin of a grasshopper that is able to exploit a broader range of host plants than either of its putative parents

Individual shell measurements at each transect site

Shell measurements (shell height, shell width) and the spire index (shell height/shell width) at each site. Data collected with calipers accurate to 0.1 mm

Estimates of habitat variables at each site

Field measurements of the local habitat at each sample site