The effect of biogeographic and phylogeographic barriers on gene flow in the brown smoothhound shark, Mustelus henlei, in the northeastern Pacific

Abstract

We assessed the effects of the prominent biogeographic (Point Conception and the Peninsula of Baja California) and phylogeographic barriers (Los Angeles Region) of the northeastern Pacific on the population connectivity of the brown smoothhound shark, Mustelus henlei (Triakidae). Data from the mitochondrial control region and six nuclear microsatellite loci revealed significant population structure among three populations: northern (San Francisco), central (Santa Barbara, Santa Catalina, Punta Lobos, and San Felipe), and southern (Costa Rica). Patterns of long-term and contemporary migration were incongruent, with long-term migration being asymmetric and occurring in a north to south direction and a lack of significant contemporary migration observed between localities with the exception of Punta Lobos that contributed migrants to all localities within the central population. Our findings indicate that Point Conception may be restricting gene flow between the northern and central populations whereas barriers to gene flow within the central population would seem to be ineffective; additionally, a contemporary expansion of tropical M. henlei into subtropical and temperate waters may have been observed.

Keywords: Microsatellite; Triakidae; mitochondria; phylogeography; shark.

Figure 1

Mustelus henlei.

Figure 2

Median-joining network of mtCR haplotypes for Mustelus henlei. Circles represent individual haplotypes with size proportional to frequency, branches indicate mutations, and black circles are hypothetical ancestors. Localities are as follows: San Francisco Bay (Blue), Santa Barbara (Yellow), Santa Catalina Island (Green), Punta Lobos (Gray), San Felipe (Red), and Costa Rica (White). Numbers of samples per circle size are represented at bottom right.

Figure 3

Population assignment of Mustelus henlei as estimated by STRUCTURE. Each individual is represented as a single histogram with percentage of ancestry on the y-axis and populations on the x-axis (SF = San Francisco, SB = Santa Barbara, SC = Santa Catalina, PL = Punta Lobos, GOC = San Felipe, and CR = Costa Rica). Spaces have been inserted between sample localities.

Figure 4

Long-term and contemporary patterns of gene flow (N_em) for Mustelus henlei. Localities are labeled as follows: San Francisco (SF), Santa Barbara (SB), Santa Catalina (SC), Punta Lobos (PB), San Felipe (GC), and Costa Rica (CR). Thin lines depict the results of MIGRATE-N analyses and the general direction of gene flow as averaged across localities from the point of origin. Estimates of N_em with credible sets (CS) not overlapping 0 are provided on either side of lines (mtCR to the left and microsatellites to the right). Bold lines depict the direction of gene flow as estimated by BAYESASS with N_em values with CS's not including 0 provided.