Incomplete reproductive isolation and low genetic differentiation despite floral divergence across varying geographic scales inCastilleja

Abstract

Divergence depends on the strength of selection and frequency of gene flow between taxa, while reproductive isolation relies on mating barriers and geographic distance. Less is known about how these processes interact at early stages of speciation. Here, we compared population-level differentiation in floral phenotype and genetic sequence variation among recently diverged Castilleja to explore patterns of diversification under different scenarios of reproductive isolation. METHODS:Using target enrichment enabled by the Angiosperms353 probe set, we assessed genetic distance among 50 populations of four Castilleja species. We investigated whether patterns of genetic divergence are explained by floral trait variation or geographic distance in two focal groups: the widespread C. sessiliflora and the more restricted C. purpurea species complex. RESULTS:We document that C. sessiliflora and the C. purpurea complex are characterized by high diversity in floral color across varying geographic scales. Despite phenotypic divergence, groups were not well supported in phylogenetic analyses, and little genetic differentiation was found across targeted Angiosperms353 loci. Nonetheless, a principal coordinate analysis of single nucleotide polymorphisms revealed differentiation within C. sessiliflora across floral morphs and geography and less differentiation among species of the C. purpurea complex. CONCLUSIONS:Patterns of genetic distance in C. sessiliflora suggest species cohesion maintained over long distances despite variation in floral traits. In the C. purpurea complex, divergence in floral color across narrow geographic clines may be driven by recent selection on floral color. These contrasting patterns of floral and genetic differentiation reveal that divergence can arise via multiple eco-evolutionary paths.

Pollinator mosaics mirror floral trait divergence within and between species ofCastilleja

“…Pollinator mosaics among the species of the Castilleja purpurea complex-We explored whether variation in floral visitors underlies floral divergence among species within the C. purpurea species complex. These species exhibited divergence in floral color across narrow geographic clines despite low genetic differentiation (Wenzell et al, 2021), which is consistent with other examples of pollinator-mediated selection driving floral color transitions (Streisfeld and Kohn, 2005;Stankowski et al, 2017). By investigating whether adaptation to distinct pollinators could be a driver of floral divergence, we found compelling support for this hypothesis in the predominately hummingbird-pollinated C. lindheimeri, some support in the largely bee-pollinated C. citrina, but less clear evidence in C. purpurea, which hosted a wide diversity of visitor groups, consistent with a generalist pollination mode.…”

“…Beyond this, C. sessiliflora exhibits intraspecific floral trait variation, including latitudinal variation in inflorescence color across its range and more divergent floral morphs in the south, which vary in color and corolla length (Fig. 1; Wenzell et al, 2021) and which occur in regions adjacent to members of the C. purpurea complex species (discussed below). In our study, floral trait variation across the distribution of C. sessiliflora corresponded to range-wide variation in floral visitors.…”

“…Data Collection Floral traits-Floral traits were measured at 23 focal populations and included 5 morphological traits (corolla length, corolla width, petaloid lip length, stigma exsertion, and bract lobe width) and inflorescence color (Royal Horticultural Society (RHS) floral color charts). Traits were measured from two flowers each of 30 plants in a single year at each population and are described in greater detail in Wenzell, et al (2021). To quantify floral color, RHS color codes were converted to Red-Green-Blue (RGB) values, followed by nonmetric multidimensional scaling (NMDS) of RGB values, using Gower distances in R package vegan (Oksanen et al, 2019), to make color values more easily interpretable.…”

Target Capture Reveals the Complex Origin of Vietnamese Ginseng

“…An advantage of this approach is that the reduced representation library approach of target capture reduces the total sequencing costs of the project, while at the same time generating re-usable data for other studies employing this kit. Moreover, this kit has been used for population level study (Van Andel et al, 2019 ; Wenzell et al, 2021 ). We prepared and pooled 319 equimolar libraries in 21 capture reactions with an average 250 ng of input DNA per pool.…”

Exploring Angiosperms353: An open, community toolkit for collaborative phylogenomic research on flowering plants

“…It also alleviates the need for bioinformatic expertise in probe design, levelling the playing field for those seeking to make use of targeted sequence capture approaches. In addition, early evidence suggests that Angiosperms353 genes, if analyzed appropriately, can be phylogenetically informative across multiple taxonomic scales (including at the population level; Beck et al, 2021 [Preprint]; Slimp et al, 2021; Wenzell et al, 2021). Broad uptake of Angiosperms353 is now being reported at conferences (e.g., Lagomarsino and Jabaily, 2020), but to date, published empirical studies in which applications of Angiosperms353 have been fully explored remain relatively few in number (Brewer et al, 2019; Van Andel et al, 2019; Gaynor et al, 2020; Howard et al, 2020 [Preprint]; Larridon et al, 2020, 2021a, 2021b; Murphy et al, 2020; Shee et al, 2020; Baker et al, 2021; Beck et al, 2021 [Preprint]; Starr et al, 2021).…”