Investigating a hybrid mixed population leads to recognizing a new species of Arctostaphylos (Ericaceae).

Abstract

While investigating the potential for Arctostaphylos species to hybridize in the mixed populations of Point Sal and Burton Mesa in Santa Barbara County, California, we discovered that Arctostaphylos from the Nipomo Mesa (San Luis Obispo County), formerly considered a northern population of A.rudis, are genetically and morphologically distinct. We name this new taxon A.nipumu after the ytt (Northern Chumash language) word for the Nipomo Mesa region. For morphological and molecular analyses, we sampled 54 plants, focusing on A.purissima, A.rudis, and A.crustacea from multiple species and comparative single species populations. Parametric and nonparametric clustering analyses (STRUCTURE and PCA) of ddRADseq data show that Arctostaphylos from the Nipomo Mesa segregate from all other samples in the dataset. In mixed populations A.purissima and A.crustacea samples cluster with samples from other unmixed populations of the same species but A.rudis samples form two distinct clusters. One is composed of the mixed populations in Santa Barbara County, and the other consists of the Nipomo Mesa population. Additionally, the Santa Barbara County A.rudis samples are admixed in STRUCTURE analysis unlike the samples from the Nipomo Mesa. A principal component analysis of eight morphological characters shows that A.rudis individuals from Santa Barbara County tend to be phenotypically variable, occurring in a wide morphological cluster that overlaps with the tight clusters formed by A.purissima, A.crustacea, and Arctostaphylos from the Nipomo Mesa. Based on this evidence we describe the Nipomo Mesapopulation as a new species of Arctostaphylos. Given its limited and fragmented distribution we believe that A.nipumu is of critical conservation concern.