Do Differences in Latitudinal Distributions of Species and Organelle Haplotypes Reflect Thermal Reaction Norms Within the Emiliania/Gephyrocapsa Complex?

Abstract

The cosmopolitan phytoplankterEmiliania huxleyicontrasts with its closest relatives that are restricted to narrower latitudinal bands, making it interesting for exploring how alternative outcomes in phytoplankton range distributions arise. Mitochondrial and chloroplast haplogroups withinE. huxleyiare shared with their closest relatives: SomeE. huxleyishare organelle haplogroups withGephyrocapsa parvulaandG. ericsoniiwhich inhabit lower latitudes, while otherE. huxleyishare organelle haplogroups withG. muellerae, which inhabit high latitudes. We investigated whether the phylogeny ofE. huxleyiorganelles reflects environmental gradients, focusing on the Southeast Pacific where the different haplogroups and species co-occur. There was a high congruence between mitochondrial and chloroplast haplogroups withinE. huxleyi. Haplogroup II ofE. huxleyiis negatively associated with cooler less saline waters, compared to haplogroup I, both when analyzed globally and across temporal variability at the small special scale of a center of coastal upwelling at 30° S. A new mitochondrial haplogroup Ib detected in coastal Chile was associated with warmer waters. In an experiment focused on inter-species comparisons, laboratory-determined thermal reaction norms were consistent with latitudinal/thermal distributions of species, withG. oceanicaexhibiting warm thermal optima and tolerance andG. muelleraeexhibiting cooler thermal optima and tolerances.Emiliania huxleyihaplogroups I and II tended to exhibit a wider thermal niche compared to the otherGephyrocapsa, but no differences among haplogroups withinE. huxleyiwere found. A second experiment, controlling for local adaptation and time in culture, found a significant difference betweenE. huxleyihaplogroups. The difference between I and II was of the expected sign, but not the difference between I and Ib. The differences were small (≤1°C) compared to differences reported previously withinE. huxleyiby local adaptation and even in-culture evolution. Haplogroup Ib showed a narrower thermal niche. The cosmopolitanism ofE. huxleyimight result from both wide-spread generalist phenotypes and specialist phenotypes, as well as a capacity for local adaptation. Thermal reaction norm differences can well explain the species distributions but poorly explain distributions among mitochondrial haplogroups withinE. huxleyi. Perhaps organelle haplogroup distributions reflect historical rather than selective processes.