Abstract
BackgroundThe glacial and interglacial cycles that characterized the Quaternary greatly affected the distribution and genetic diversity of plants. In the Neotropics, few phylogeographic studies have focused on coastal species outside of the Atlantic Rainforest. Climatic and sea level changes during the Quaternary played an important role in the evolutionary history of many organisms found in coastal regions. To contribute to a better understanding of plant evolution in this environment in Southern South America, we focused on Calibrachoa heterophylla (Solanaceae), an endemic and vulnerable wild petunia species from the South Atlantic Coastal Plain (SACP).ResultsWe assessed DNA sequences from two cpDNA intergenic spacers and analyzed them using a phylogeographic approach. The present phylogeographic study reveals the influence of complex geologic and climatic events on patterns of genetic diversification. The results indicate that C. heterophylla originated inland and subsequently colonized the SACP; the data show that the inland haplogroup is more ancient than the coastal one and that the inland was not affected by sea level changes in the Quaternary. The major diversification of C. heterophylla that occurred after 0.4 Myr was linked to sea level oscillations in the Quaternary, and any diversification that occurred before this time was obscured by marine transgressions that occurred before the coastal sand barrier’s formation. Results of the Bayesian skyline plot showed a recent population expansion detected in C. heterophylla seems to be related to an increase in temperature and humidity that occurred at the beginning of the Holocene.ConclusionsThe geographic clades have been formed when the coastal plain was deeply dissected by paleochannels and these correlate very well with the distributional limits of the clades. The four major sea transgressions formed a series of four sand barriers parallel to the coast that progressively increased the availability of coastal areas after the regressions and that may have promoted the geographic structuring of genetic diversity observed today. The recent population expansion for the entire species may be linked with the event of marine regression after the most recent sea transgression at ~5 kya.
Highlights
The glacial and interglacial cycles that characterized the Quaternary greatly affected the distribution and genetic diversity of plants
The Bayesian chronogram indicated the Mainland haplogroup was the oldest (0.77 Myr), nearly twice the age of the remaining haplogroups (Figure 4). These results suggest that inland populations may have given rise to the current haplogroups found in the South Atlantic Coastal Plain (SACP) by a colonization route from west to east, which was proposed for flora migration from Andean region to Atlantic Coastal in South America [25]
A major transgression event occurred around this time where the sea level rose >20 m and most of the lowlands on the coast were submerged [45]. This may have eliminated much of the diversity already present (Figure 6a). This and the three subsequent major transgressions formed a series of four sand barriers parallel to the coast that progressively increased the availability of coastal areas after the regressions and may have allowed for the genetic diversity observed today
Summary
The glacial and interglacial cycles that characterized the Quaternary greatly affected the distribution and genetic diversity of plants. In South America, fully formed glaciers occurred only in the Andes, and the main climatic effects on most of the region were that closed vegetation types (forests) alternated with open formations (grasslands) between the glacial and interglacial periods [3]. This might have resulted in more complex histories regarding the establishment and composition of current vegetation than simple extinctions of local biota and the formation of typical glacial refugia. The accentuated moisture in this phase was reported from the South Atlantic Coastal Plain (SACP) and adjacent area [13,14] According to these references, the pollen concentration of individual taxa increased dramatically at the beginning of the Holocene, providing evidence for rapid climatic change
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