Meeting Abstract
32.3 Sunday, Jan. 5 08:30 Problems with dating a nudibranch: age and ancestral range reconstruction of the genus Acanthodoris Gray, 1850 (Mollusca, Gastropoda, Nudibranchia) HALLAS, JM*; GOSLINER, TM; San Francisco State University; California Academy of Sciences jhallas@calacademy.org
In the absence of a reliable fossil record, the use of geological events to calibrate a relaxed molecular clock has been widely employed. However, the placement of such events on a phylogenetic tree can be problematic in accurately dating lineages. The unique distribution of the nudibranch genus Acanthodoris Gray, 1850 allows for an examination of the use of calibration points despite a fossil record, and their effect on ancestral range reconstruction (ARR). For this study, a phylogeny of Acanthodoris was estimated from a dataset of three genetic markers: 16s, cytochrome c oxidase 1 (COI), and histone 3 (H3), using maximum likelihood and Bayesian inference. To estimate the age and ancestral ranges of Acanthodoris, two independent analyses were conducted using a relaxed molecular clock in BEAST and the dispersal-extinction cladogenesis (DEC) model in LAGRANGE. The opening of the Bering Strait (5.4-5.5 Mya) and the formation of the Baja California Peninsula (~5.5 Mya) were used as calibration points in both analyses, but one had an additional calibration date of 95-105 Mya, which coincides with the spilt of Africa from South America. As expected the analysis which utilized the spit between Africa and South America placed the age of Acanthodoris much older than the analysis that did not, with a difference of ~90 Mya. However, both analyses gave similar dispersal scenarios for the common ancestor of Acanthodoris originating in the Atlantic. The ARR that lacked the older calibration point suggests that there were two separate dispersals from the northwestern Atlantic, with one dispersal event into the northeastern Pacific. In contrast, the older analysis proposes two independent colorizations of the northeastern Pacific as well as an extinction event in the northeastern Atlantic followed its recolonization.
