A Potential Case of Reinforcement in a Facultatively Sexual Unicellular Eukaryote

Selection on sporulation strategies in a metapopulation can lead to coexistence

In constant environments, the coexistence of similar species or genotypes is generally limited. In a metapopulation context, however, types that utilize the same resource but are distributed along a competition-colonization trade-off can coexist. Prior work used a generic trade-off between within-deme competitive ability and between-deme dispersal ability. We show that sporulation in yeasts and other microbes can create a natural trade-off such that strains that initiate sporulation at higher rates suffer in terms of within-deme competition but benefit in terms of between deme dispersal. Using chemostat dynamics within patches, we first show that the rate of sporulation determines the colonization ability of the strain, with colonization ability increasing with sporulation rate up to a point. Metapopulation stability of a single strain exists in a defined range of sporulation rates. We pairwise invasability plots to show that coexistence of strains with different sporulation rates generally occurs, but that the set of sporulation rates that can potentially coexist is smaller than the set that allows for stable metapopulations. We also show how a continuous set of strains can coexist and verify our conclusions with numerical calculations and stochastic simulations. Stable variation in sporulation rates is expected under a wide range of ecological conditions.

Defining and Disrupting Species Boundaries in Saccharomyces

The genus Saccharomyces is an evolutionary paradox. On the one hand, it is composed of at least eight clearly phylogenetically delineated species; these species are reproductively isolated from each other, and hybrids usually cannot complete their sexual life cycles. On the other hand, Saccharomyces species have a long evolutionary history of hybridization, which has phenotypic consequences for adaptation and domestication. A variety of cellular, ecological, and evolutionary mechanisms are responsible for this partial reproductive isolation among Saccharomyces species. These mechanisms have caused the evolution of diverse Saccharomyces species and hybrids, which occupy a variety of wild and domesticated habitats. In this article, we introduce readers to the mechanisms isolating Saccharomyces species, the circumstances in which reproductive isolation mechanisms are effective and ineffective, and the evolutionary consequences of partial reproductive isolation. We discuss both the evolutionary history of the genus Saccharomyces and the human history of taxonomists and biologists struggling with species concepts in this fascinating genus.

Harnessing the power of digital droplet PCR to conduct real-world microbial competitions

The budding yeast, Saccharomyces cerevisiae, has a long and storied history as a model organism for genetic, cellular and molecular biological research. More recently, researchers have sought to understand the ecology and evolution of its sister species, Saccharomyces paradoxus, in part to put our vast knowledge of the model yeast into its natural context (Replansky et al. ). However, the research tools have been limited, and most investigations into natural populations have either been descriptions of patterns of biogeography or taken the organism back into the laboratory for mating, growth and competition assays (Kuehne et al. ; Miller & Greig ; Murphy & Zeyl ; Samani et al. ). The link between what occurs out in the real world and what is measured in the laboratory has not yet been made, as so much is still unknown about the natural history of these yeasts. In this issue of Molecular Ecology Resources, Boynton et al. () take a major step towards bridging laboratory studies with field ecological research. By isolating a panel of S. paradoxus strains from a wooded area, culturing them in the laboratory, reintroducing pairs back into their habitat on natural substrate and monitoring the frequency of individual strains using digital droplet PCR, the researchers were able to use the framework of laboratory-based microbial competitions, but conduct them in a natural setting. While there is still more to learn about how to optimize this approach, it represents an exciting step in microbial ecological research and should prove an important tool for other species and numerous ecological questions.

Gene exchange between two divergent species of the fungal human pathogen, Coccidioides

The fungal genus Coccidioides is composed of two species, Coccidioides immitis and Coccidioides posadasii. These two species are the causal agents of coccidioidomycosis, a pulmonary disease also known as valley fever. The two species are thought to have shared genetic material due to gene exchange in spite of their long divergence. To quantify the magnitude of shared ancestry between them, we analyzed the genomes of a population sample from each species. Next, we inferred what is the expected size of shared haplotypes that might be inherited from the last common ancestor of the two species and find a cutoff to find what haplotypes have conclusively been exchanged between species. Finally, we precisely identified the breakpoints of the haplotypes that have crossed the species boundary and measure the allele frequency of each introgression in this sample. We find that introgressions are not uniformly distributed across the genome. Most, but not all, of the introgressions segregate at low frequency. Our results show that divergent species can share alleles, that species boundaries can be porous, and highlight the need for a systematic exploration of gene exchange in fungal species.

No evidence for extrinsic post-zygotic isolation in a wild Saccharomyces yeast system

Although microorganisms account for the largest fraction of Earth's biodiversity, we know little about how their reproductive barriers evolve. Sexual microorganisms such as Saccharomyces yeasts rapidly develop strong intrinsic post-zygotic isolation, but the role of extrinsic isolation in the early speciation process remains to be investigated. We measured the growth of F₁ hybrids between two incipient species of Saccharomyces paradoxus to assess the presence of extrinsic post-zygotic isolation across 32 environments. More than 80% of hybrids showed either partial dominance of the best parent or over-dominance for growth, revealing no fitness defects in F₁ hybrids. Extrinsic reproductive isolation therefore likely plays little role in limiting gene flow between incipient yeast species and is not a requirement for speciation.

Reinforcement's incidental effects on reproductive isolation between conspecifics

Reinforcement—the process whereby maladaptive hybridization leads to the strengthening of prezygotic isolation between species—has a long history in the study of speciation. Because reinforcement affects traits involved in mate choice and fertility, it can have indirect effects on reproductive isolation between populations within species. Here we review examples of these “cascading effects of reinforcement” (CER) and discuss different mechanisms through which they can arise. We discuss three factors that are predicted to influence the potential occurrence of CER: rates of gene flow among populations, the strength of selection acting on the traits involved in reinforcement, and the genetic basis of those traits. We suggest that CER is likely if (1) the rate of gene flow between conspecific populations is low; (2) divergent selection acts on phenotypes involved in reinforcement between sympatric and allopatric populations; and (3) the genetic response to reinforcement differs among conspecific populations subject to parallel reinforcing selection. Future work continuing to address gene flow, selection, and the genetic basis of the traits involved in the reinforcement will help develop a better understanding of reinforcement as a process driving the production of species diversity, both directly and incidentally.