Two cryptic species of California mustard within Caulanthus lasiophyllus

Abstract

Premise: Cryptic species are evolutionarily distinct lineages lacking distinguishing morphological traits. Hidden diversity may be lurking in widespread species whose distributions cross phylogeographic barriers. This study investigates molecular and morphological variation in the widely distributed Caulanthus lasiophyllus (Brassicaceae) in comparison to its closest relatives.

Methods: Fifty-two individuals of C. lasiophyllus from across the species' range were sequenced for the nuclear ribosomal internal transcribed spacer region (ITS) and the chloroplast trnL-F region. A subset of these samples were examined for the chloroplast ndhF gene. All 52 individuals were scored for 13 morphological traits, as well as monthly and annual climate conditions at the collection locality. Morphological and molecular results are compared with the closest relatives-C. anceps and C. flavescens-in the "Guillenia Clade." To test for polyploidy, genome size estimates were made for four populations.

Results: Caulanthus lasiophyllus consists of two distinct lineages separated by eight ITS differences-eight times more variation than what distinguishes C. anceps and C. flavescens. Fewer variable sites were detected in trnL-F and ndhF regions, yet these data are consistent with the ITS results. The two lineages of C. lasiophyllus are geographically and climatically distinct; yet morphologically overlapping. Their genome sizes are not consistently different.

Conclusions: Two cryptic species within C. lasiophyllus are distinguished at the molecular, geographic, and climatic scales. They have similar genome sizes and are morphologically broadly overlapping, but an ephemeral basal leaf character may help distinguish the species.

Keywords: Brassicaceae; California mustard; annual precipitation; chloroplast ndhF gene; chloroplast trnL-F region; internal transcribed spacer region; mean temperature; morphological analysis.

Figure 1

The location and type of C. lasiophyllus samples in California. Circles = northern/coastal type, triangles = southern/inland type, and stars = putative hybrids. Numbers inside symbols indicate the haplotype identifier (see Table 1). Four southern/inland samples outside of California are not shown (two from the inland regions of northern Baja California, Mexico—haplotype 4; one from southwestern Arizona—haplotype 5; one from southwestern Nevada—haplotype 4). Arrows point to the locations where the two types of C. lasiophyllus are geographically adjacent (distance between sampled populations indicated in kilometers).

Figure 2

One of 16 equally parsimonious trees using unique haplotypes from the combined ITS, trnL‐F, and ndhF regions. The unique haplotype number of the northern/coastal type (circles) and southern/inland type (triangles) follow Table 3. Branch length is proportional to the number of substitutions. Bootstrap percentages are indicated above the branches. The tree is rooted with Streptanthus glandulosus.

Figure 3

Multidimensional scaling plot for the first two axes representing differences for 13 morphological characters for the two types of C. lasiophyllus (filled circles = northern/coastal; open triangles = southern/inland). The minimum stress value (0.01027) indicates excellent representation of the differences in the ordination. The two types are significantly differentiated (ANOSIM, Euclidean distances, 9999 permutations, R statistic = 0.1074, p = 0.009).

Figure 4

Principal component analysis of 13 morphological traits for the two types of C. lasiophyllus. Filled circles are northern/coastal types and triangles are southern/inland types. In (A), principal component axis one (PC1) represents 33.6% of the variance and PC2 explains 14.6% of the variance. In (B), we compare PC2 and PC4, the latter explains an additional 11.7% of the variation. Putative hybrids were not included in the PCA of morphological data because the parental types are not morphologically differentiated.

Figure 5

Young basal rosette leaves differentiate the northern/coastal and southern/inland lineages. Each leaf came from a separate individual. Two to three individuals per population for two populations per lineage are shown. Populations are further described in Appendix I.

Figure 6

Multidimensional scaling plot for the first two axes representing differences in 30‐year normals for mean monthly temperature and total monthly precipitation between the two types of C. lasiophyllus (filled circles = northern/coastal; open triangles = southern/inland). Two samples from Baja, Mexico were not included because of lack of PRISM data. The minimum stress value (0.01738819) indicates excellent representation of the differences in the ordination. The two types are significantly differentiated (ANOSIM, Euclidean distances, 9999 permutations, R statistic = 0.4178, p < 0.0001).