Palaeoclimatic conditions in the Mediterranean explain genetic diversity of Posidonia oceanica seagrass meadows

Abstract

Past environmental conditions in the Mediterranean Sea have been proposed as main drivers of the current patterns of distribution of genetic structure of the seagrass Posidonia oceanica, the foundation species of one of the most important ecosystems in the Mediterranean Sea. Yet, the location of cold climate refugia (persistence regions) for this species during the Last Glacial Maximum (LGM) is not clear, precluding the understanding of its biogeographical history. We used Ecological Niche Modelling together with existing phylogeographic data to locate Pleistocene refugia in the Mediterranean Sea and to develop a hypothetical past biogeographical distribution able to explain the genetic diversity presently found in P. oceanica meadows. To do that, we used an ensemble approach of six predictive algorithms and two Ocean General Circulation Models. The minimum SST in winter and the maximum SST in summer allowed us to hindcast the species range during the LGM. We found separate glacial refugia in each Mediterranean basin and in the Central region. Altogether, the results suggest that the Central region of the Mediterranean Sea was the most relevant cold climate refugium, supporting the hypothesis that long-term persistence there allowed the region to develop and retain its presently high proportion of the global genetic diversity of P. oceanica.

Figure 1

Sea surface temperatures (SST) at which Posidonia oceanica meadows are found in the Mediterranean Sea according to the locations compiled in this study. The figure illustrates the high seasonality between summer and winter. Fewer presence records occur in marginal conditions of SST below 10 °C or above 28 °C.

Figure 2

Distribution of Posidonia oceanica predicted by ensembles under current (a) and Last Glacial Maximum (LGM) conditions (b). A major probability of occurrence is predicted in the Western and Northern Mediterranean Sea for the present. Three putative glacial refugia, located in the Southern Mediterranean, were found by hindcasting to the LGM. Divisions correspond to genetic regions identified in the Mediterranean Sea by ref. and ref. . Maps were generated using R.

Figure 3

Distribution of probability of presence predicted for Posidonia oceanica under current and Last Glacial Maximum (LGM) conditions. Globally, climatic conditions in the Mediterranean Sea were less suitable for P. oceanica during the LGM than in the present. The white dot represents the median and the green bar represents the interquartile range.

Figure 4

Comparison among the genetic diversity found by (a) ref. and (b) ref. in the different regions of the Mediterranean Sea, and the probability of occurrence of Posidonia oceanica found by this study during the Last Glacial Maximum (LGM) (c) and present conditions (d). Central regions seem to have been relevant glacial refugia according to the high values found both for allelic richness (A) and the probability of occurrence during the LGM. PA: private alleles; C. I. and C. II.: Central I and Central II subregions of the Central Mediterranean found by ref. .

Figure 5

Occurrence data used for ecological niche models of Posidonia oceanica (grey circles), and genetic regions identified in the Mediterranean Sea by ref. (Study 1): Western, Central and Eastern (populations in orange squares); and ref. (Study 2): Western, Central I (C. I), Central II (C. II), and Eastern (populations in green triangles). The map was generated using R.