Eukaryotic microbes, species recognition and the geographic limits of species: examples from the kingdom Fungi

Abstract

The claim that eukaryotic micro-organisms have global geographic ranges, constituting a significant departure from the situation with macro-organisms, has been supported by studies of morphological species from protistan kingdoms. Here, we examine this claim by reviewing examples from another kingdom of eukaryotic microbes, the Fungi. We show that inferred geographic range of a fungal species depends upon the method of species recognition. While some fungal species defined by morphology show global geographic ranges, when fungal species are defined by phylogenetic species recognition they are typically shown to harbour several to many endemic species. We advance two non-exclusive reasons to explain the perceived difference between the size of geographic ranges of microscopic and macroscopic eukaryotic species when morphological methods of species recognition are used. These reasons are that microbial organisms generally have fewer morphological characters, and that the rate of morphological change will be slower for organisms with less elaborate development and fewer cells. Both of these reasons result in fewer discriminatory morphological differences between recently diverged lineages. The rate of genetic change, moreover, is similar for both large and small organisms, which helps to explain why phylogenetic species of large and small organisms show a more similar distribution of geographic ranges. As a consequence of the different rates in fungi of genetic and morphological changes, genetic isolation precedes a recognizable morphological change. The final step in speciation, reproductive isolation, also follows genetic isolation and may precede morphological change.

Paracoccidioides brasiliensis: phylogenetic and ecological aspects

“…However, such recognition does not apply to asexual populations, or even to sexual ones, and it may lead to conflict between the potential and real gene flows. It is important to note that even allopatric species may retain their interbreeding capacity, and furthermore, there is no selection for gene-flow barriers since the species are geographically isolated [81]. The ecological concept of species defines species as a group of individuals that occupy the same niche and this concept is closely connected to the biological one because natural selection will favor the individuals that interbreed only with those that have the same ecological adaptation.…”

“…It permits inference of the reproductive mode of the different divergent species, even without the actual production of sexual structures. Additionally, in a single species, clonal and recombining reproduction can be temporally or spatially separated [81]. Consequently, the failure to detect sexual structure does not signify the absence of such structures.…”

“…If the interbreeding between them is not positively selected because of the low adaptive value of the hybrids, there will be a reinforcement of reproductive isolation by natural selection [40]. This reinforcement is very important because speciation involves not only divergence, but also maintenance, mainly when the derived species coexist [81]. Dettman et al [87] showed how these barriers to inter-species interbreeding may be different between allopatric and sympatric species of Neurospora; they observed a higher reproductive success for allopatric heterospecific pairings than for sympatric ones.…”

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“…Since the use of molecular data in fungal taxonomy the relative relevance of morphology, niche and DNA phylogenies have been debated (Carbone and Kohn, 2001;Taylor et al, 2006). Species should be defined according to the evolutionary species concept ''An entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary tendencies and historical fate'' (Wiley and Mayden, 2000).…”

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“…The first part of this postulate, which has long been debated, implies that if microorganisms form worldwide populations they should not be considered as part of the threatened biodiversity. However, it is now well established that not all microorganisms are ubiquitous [9,10]. The second part of the Baas Becking's hypothesis implies that microbes could freely adapt in response to environmental changes.…”