February 1st to 5th 2016
Olhão, Portugal
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Recent demographic changes at a rear edge of giant kelp (Macrocystis pyrifera) did not reduce the overall levels of genetic diversity of extant populations

Oral Presentation
Future Oceans
Wednesday, February 3, 2016 -
11:15 to 11:30

Assis, J. 1 Ladah, L. 2 Bell, T. 3 Cavanaugh, K. 4 Babuder, M. 5 Serrão, E.A. 6

1Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
2Department of Biological Oceanography, CICESE
3Earth Research Institute, University of California, Santa Barbara, Santa Barbara, CA, USA
4Department of Geography, University of California, Los Angeles, Los Angeles, CA, USA
5Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
6Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal

The negative outcomes of recent climate change may be particularly evident in extant populations at distributional edges. There, limiting environmental conditions can reduce population sizes and cause erosion of genetic diversity that may prevail over past evolutionary processes, reducing evolvability. This study aimed to test whether populations of giant kelp (Macrocystis pyrifera) from two regions close to a distributional edge (Baja California; Pacific Ocean) experienced range contraction and recent genetic erosion due to ongoing climate change. Canopy abundance was analysed from 1992 to 2014 using satellite-based methods. Levels of genetic diversity and differentiation were inferred by genotyping ~400 individuals for seven microsatellite loci. Signatures of bottlenecks were further evaluated by testing for reductions in M-ratios, departures from equilibrium in heterozygosity and by contrasting different scenarios of population history using Approximate Bayesian Computations (ABC). Our findings show large variance in abundance, primarily linked to positive anomalies in sea surface temperatures. These produced more detrimental effects closer to the edge, where range reduction bordered extinction for different periods. Genetic analyses supported the two regions as separate groups, both with private diversity and low differentiation. The southern edge displayed lower diversity, yet all tests failed to provide evidence for bottlenecks. Furthermore, the ABC indicated that changes in population sizes could only have occurred >1000 generations ago, when both regions likely diverged. Thus, we propose that other processes like founder effects and/or random drift are predominant in shaping the genetic diversity at this rear edge, and that despite strong demographic changes, giant kelp might sustain large population sizes by persisting, possibly in deeper reefs, unaffected by the warming trends, or in persistent microscopic development stages.
keywords: 
climate change, rear edge, range shift, genetic diversity, kelp, Macrocystis pyrifera

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