GENOMIC DATA CLARIFY IDENTITIES AND ORIGINS OF PURPORTED HYBRIDS AND ANOMALOUS OAKS ON THE CHANNEL ISLANDS OF CALIFORNIA

Hybridization is common among plant taxa and can lead to adaptive introgression, range expansion, and even island colonization. Patterns of hybridization in natural and disturbed populations can reflect a species' evolutionary history and elucidate patterns of human influence. On the Channel Islands, reports of hybrid oaks, such as Q. × macdonaldii Greene & Kellogg, and anomalous individuals that appear to be oaks typically found on the mainland, including Q. lobata Née, Q. douglasii Hook. & Arn., and Q. engelmannii Greene, raise questions about their origin and history on the islands. Here, we examine the validity of those reports by using nuclear and chloroplast whole genome sequence data to elucidate species identities. As expected, we find that Q. × macdonaldii individuals are hybrids between Q. lobata and the endemic Island Scrub Oak, Q. pacifica Nixon & C.H. Mull., but identify only one pure Q. lobata individual on the islands, whose chlorotype is related to trees in the Santa Ynez Valley of Santa Barbara County, CA. In addition, the trees thought to be Q. douglasii and Q. engelmannii are actually Q. pacifica × Q. douglasii and Q. pacifica × Q. engelmannii hybrids, respectively. These hybrids were determined to be F1 hybrids that exclusively have the chlorotype of Q. pacifica, indicating unidirectional hybridization with the Island Scrub Oak as the maternal parent. The lack of pure parental individuals raises the questions of where the pollen donors came from, when were they present on the islands, and why have they disappeared. The lack of backcrossed individuals or an F2 generation suggests the mainland species may not be adapted to Channel Island environments and probably do not represent an ancient population of these species on the islands. Instead, these findings are consistent with a more recent arrival, potentially supporting the notion that Indigenous Peoples brought the mainland species to the Channel Islands. Conservation managers should be aware of the presence of hybrid individuals that are morphologically indistinguishable from their parental species and consult Indigenous groups on the preservation of mainland species and their hybrids.