Using environmental niche models to test the 'everything is everywhere' hypothesis for Badhamia

Abstract

It is often discussed whether the biogeography of free-living protists is better explained by the 'everything is everywhere'(EiE) hypothesis, which postulates that only ecology drives their distribution, or by the alternative hypothesis of 'moderate endemicity' in which geographic barriers can limit their dispersal. To formally test this, it would be necessary not only to find organisms restricted to a geographical area but also to check for their presence in any other place with a similar ecology. We propose the use of environmental niche models to generate and test null EiE distributions. Here we have analysed the distribution of 18S rDNA variants (ribotypes) of the myxomycete Badhamia melanospora (belonging to the protozoan phylum Amoebozoa) using 125 specimens from 91 localities. Two geographically structured groups of ribotypes congruent with slight morphological differences in the spores can be distinguished. One group comprises all populations from Argentina and Chile, and the other is formed by populations from North America together with human-introduced populations from other parts of the world. Environmental climatic niche models constructed separately for the two groups have significant differences, but show several overlapping areas. However, only specimens from one group were found in an intensively surveyed area in South America where both niche models overlap. It can be concluded that everything is not everywhere for B. melanospora. This taxon constitutes a complex formed by at least two cryptic species that probably diverged allopatrically in North and South America.

Figure 1

Phylogenetic analyses. (a): Fifty percent majority-rule rooted consensus tree of a 533 bp fragment of the small subunit rDNA (SSU) of 125 specimens of B. melanospora obtained by Bayesian inference. Triangles represent collapsed groups of samples with the same ribotype and geographic origin. Colours indicate the origin of the specimens. The scale bar represents evolutionary distance in changes per site. Bayesian posterior probabilities and RAxML support values are at each node. (b): Ribotype network of a 533 bp fragment of small subunit rDNA (SSU) of 123 specimens of B. melanospora. Circle size is proportional to the number of specimens within each ribotype, and dots between ribotypes represent unobserved, inferred ancestral ribotypes. Lines between ribotypes represent mutational steps between alleles. Colours denote specimen origin.

Figure 2

Variability in spore morphology. A total of 32 specimens from ribotype groups A and B were randomly selected for morphological studies. Ten spores per specimen were observed and measured (a): (s.e.m) pictures displaying one representative spore from each studied specimen. (b): Box-and-whiskers plot of the diameter of the spores in μm.

Figure 3

Environmental Niche Models. (a): Predictive ecological models based on the Maxent algorithm. Probabilities of presence >0.5 are represented for ribotype groups A (green) and B (red). Overlapping areas are shown in black. (b): Close-up of overlapping niche areas in South America. Yellow dots denote localities where specimens from ribotype group A were found. (c): Niche comparisons of ribotype groups A and B based on D and I as measures of niche overlap. Dotted lines represent niche overlap measures of the original data, and bars show the expected degree of niche overlap when samples are drawn from the same distribution (i.e., pooled samples of occurrence points from the ribotype groups).