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bpwhite:mol_evolution_snails_lake

Molecular Evolution and Hydrographic History of Gastropods

Author: Bryan P. White

Original Publication: 10/11/2012

Edited for grammar/syntax: 07/31/2019

Introduction

Lake Titicaca sits on the border of Bolivia and Peru in South America. The lake originated 2-3 million years ago during the Late Pliocene/Early Pleistocene and underwent cyclical expansion and contractions due to glacial-interglacial cycles. The greatest maximum north-to-south distance of the lake was approximately 800 km long. It has been suggested that these large-scale expansions and contractions, when coupled with large fluctuations in salinity, may have resulted in large amounts of extinction.

Large numbers of extinctions were likely as the Lake Titicaca region is comprised of 533 aquatic species, but only 64 (~12%) are thought to be endemic (originated from the lake-region itself). Compared to other ancient lake regions, those numbers are much smaller. Two hypotheses have been suggested to account for the low numbers of endemic species in the region. First, that the region was originally tropical and changes in elevation due to geological uplifting caused extinctions as the tropical species were unable to cope with higher elevations, and second, that large changes in water chemistry resulted in these extinctions.

There are 14 species of Heleobia, family Cochliopidae, in the Lake Titicaca region that may lend themselves to answering several questions about the biogeographical history of Lake Titicaca and the Altiplano region. The first question is, did the lake serve as a reservoir for ancient species, or facilitate the recent divergence of those few endemic species? Secondly, are diversification events related to the paleohydrogeological events on the Altiplano? Thirdly, are there sub-regions within the altiplano region, reflecting the different times of these paleohydrogeological events.

Methods

Heleobia was broadly sampled throughout Lake Titicaca and the Altiplano, as well as in the Andes and Chilean coastal region, Argentina, members of the sister genus from Europe and West Asia, and two species of Heleobops from North and Central America. Specimens were sequenced for their cytochrome oxidase I (COI) gene. A strict molecular clock tree was constructed using BEAST and haplotype network constructed using TCS.

Results

The Heleobia found in Lake Titicaca are not monophyletic but rather mixed with Altiplano species. Heleobia and Semisalsinae show a split at 3.57 million years ago (mya) (with a range of 2.14 to 5.43 mya), and the Northern Heleobia split with Altiplano Helobia 0.92 mya (with a range of 0.46 to 1.52 mya). The most recent common ancestor of all Altiplano species is only 0.53 mya (with a range of 0.28 to 0.80 mya).

Deeper cladogenic events correlate tightly within hydrogeological formations of Lake Titicaca, and Heleobia represents a “flock” of species with a Lake Titicaca origin. Haplotype network analysis suggests that the originating haplotypes likely came from within Lake Titicaca, supporting the intralacustrine (within-lake) speciation model rather than the reservoir of ancient species model. Even those few taxa found outside of the lake are closely related enough to the Lake Titicaca taxa to suggest they originated from Lake Titicaca and diversified in the Altiplano. However, it is possible that some of those species were colonized from an ancestor outside of Lake Titicaca. This suggests that new species originate within the lake, disperse outside, but only recently (~500,000 years ago). Dispersal through rivers does add some confusion to these biological processes.

It’s likely that the ancestor of Heleobia was already adapted for brackish waters, which suggests that it is one of the species which survived the extinction events following changes in lake salinity. As the lake returned to freshwater conditions, it was likely recolonized by other freshwater species, which owes to its 88% non-endemic species rate.

Future studies should take into account fossil data calibrations, and a multi-population, multi-locus approach would add to the robustness of the phylogeny, but the overall conclusions of this paper should not change.

Critique

This paper offers an excellent demonstration of how a region with clear, dateable geological events owes itself to biogeography and phylogeography. I found the results and conclusions were strongly supported by their data. The only drawback of this study is likely to be the single-locus approach; however, the authors were clear to use a conservative method in molecular clock estimation which should account for the variability of node depth resultant from the single-locus, cytochrome oxidase I approach.

Literature Cited

Kroll, O., Hershler, R., Albrecht, C., Terrazas, E.M., Apaza, R., Fuentealba, C., Wolff, C. and Wilke, T., 2012. The endemic gastropod fauna of Lake Titicaca: correlation between molecular evolution and hydrographic history. Ecology and Evolution, 2(7), pp.1517-1530. doi: https://doi.org/10.1002/ece3.280

bpwhite/mol_evolution_snails_lake.txt · Last modified: 2019/07/31 17:46 by bpwhite