A Simulation Study of the Ecological Speciation Conditions in the Galician Marine Snail Littorina saxatilis

In the last years, the interest in evolutionary divergence at small spatial scales has increased and so did the study of speciation caused by ecologically based divergent natural selection. The evolutionary interplay between gene flow and local adaptation can lead to low-dispersal locally adapted specialists. When this occurs, the evolutionary interplay between gene flow and local adaptation could eventually lead to speciation. The L. saxatilis system consists of two ecotypes displaying a microhabitat-associated intraspecific dimorphism along the wave-exposed rocky shores of Galicia. Despite being a well-known system, the dynamics of the ecotype formation remain unclear and cannot be studied from empirical evidence alone. In this study, individual-based simulations were used to incorporate relevant ecological, spatial, and genetic information, to check different evolutionary scenarios that could evolve non-random mating preferences and finally may facilitate speciation. As main results, we observed the evolution of intermediate values of choice which matches the estimates from empirical data of L. saxatilis in Galician shores and coincides with previous theoretical outcomes. Also, the use of the mating correlation as a proxy for assortative mating led to spuriously inferring greater reproductive isolation in the middle habitat than in the others, which does not happen when directly considering the choice values from the simulations. We also corroborate the well-known fact that the occurrence of speciation is influenced by the strength of selection. Taken together, this means, also according to other L. saxatilis systems, that speciation is not an immediate consequence of local divergent selection and mating preferences, but a fine tuning among several factors including the ecological conditions in the shore levels, the selection strength, the mate choice stringency, and cost to choosiness. The L. saxatilis system could correspond to a case of incomplete reproductive isolation, where the choice intensity is intermediate and local adaptation within the habitat is strong. These results support previous interpretations of the L. saxatilis model system and indicate that further empirical studies would be interesting to test whether the mate choice mechanism functions as a similarity-like mechanism as has been shown in other littorinids.

Investigating In Vitro Mating Preference Between or Within the Two Forms of Pyrenophora teres and Its Hybrids

Net blotch diseases result in significant yield losses to barley industries worldwide. They occur as net-form and spot-form net blotch caused by Pyrenophora teres f. teres and P. teres f. maculata, respectively. Hybridization between the forms was proposed to be rare, but recent identifications of field hybrids has renewed interest in the frequency and mechanisms underlying hybridization. This study investigates the mating preference of P. teres f. teres, P. teres f. maculata, and laboratory-produced hybrids in vitro, using 24 different isolates and four different experimental setups. Two crosses in our study produced ascospores during two intervals separated by a 32- to 35-day period of no ascospore production. For these crosses, P. teres f. teres isolates mated with isolates of the same form during the early ascospore production interval, and produced hybrids during the later interval. P. teres f. maculata isolates did not mate with isolates of the same form, but instead hybridized with P. teres f. teres isolates. Analyses based on DArTseq markers confirmed that laboratory-produced hybrids, when given the choice to mate with both P. teres f. teres and P. teres f. maculata, mated with P. teres f. teres isolates. These results unravel a novel concept that P. teres f. teres seems to have a greater reproduction vigor than P. teres f. maculata, which could lead to increased prevalence of hybrid incidences in vivo.

Divergence in Thermal Physiology Could Contribute to Vertical Segregation in Intertidal Ecotypes of Littorina saxatilis

AbstractThermal stress is a potentially important selective agent in intertidal marine habitats, but the role that thermal tolerance might play in local adaptation across shore height has been underexplored. Northwest Spain is home to two morphologically distinct ecotypes of the periwinkle Littorina saxatilis, separated by shore height and subject to substantial differences in thermal stress exposure. However, despite other biotic and abiotic drivers of ecotype segregation being well studied, their thermal tolerance has not been previously characterized. We investigated thermal tolerance across multiple life history stages by employing the thermal death time (TDT) approach to determine (i) whether the two ecotypes differ in thermal tolerance and (ii) how any differences vary with life history stage. Adults of the two ecotypes differed in their thermal tolerance in line with their shore position: the upper-shore ecotype, which experiences more extreme temperatures, exhibited greater endurance of thermal stress compared with the lower-shore ecotype. This difference was most pronounced at the highest temperatures tested. The proximate physiological basis for these differences is unknown but likely due to a multifarious interaction of traits affecting different parts of the TDT curve. Differences in tolerance between ecotypes were less pronounced in early life history stages but increased with ontogeny, suggesting partial divergence of this trait during development. Thermal tolerance could potentially play an important role in maintaining population divergence and genetic segregation between the two ecotypes, since the increased thermal sensitivity of the lower-shore ecotype may limit its dispersal onto the upper shore and so restrict gene flow.

Assortative mating, sexual selection, and their consequences for gene flow in Littorina

When divergent populations are connected by gene flow, the establishment of complete reproductive isolation usually requires the joint action of multiple barrier effects. One example where multiple barrier effects are coupled consists of a single trait that is under divergent natural selection and also mediates assortative mating. Such multiple-effect traits can strongly reduce gene flow. However, there are few cases where patterns of assortative mating have been described quantitatively and their impact on gene flow has been determined. Two ecotypes of the coastal marine snail, Littorina saxatilis, occur in North Atlantic rocky-shore habitats dominated by either crab predation or wave action. There is evidence for divergent natural selection acting on size, and size-assortative mating has previously been documented. Here, we analyze the mating pattern in L. saxatilis with respect to size in intensively sampled transects across boundaries between the habitats. We show that the mating pattern is mostly conserved between ecotypes and that it generates both assortment and directional sexual selection for small male size. Using simulations, we show that the mating pattern can contribute to reproductive isolation between ecotypes but the barrier to gene flow is likely strengthened more by sexual selection than by assortment.

Assortative mating in sympatric ascomycete fungi revealed by experimental fertilizations

Mate recognition mechanisms resulting in assortative mating constitute an effective reproductive barrier that may promote sexual isolation and speciation. While such mechanisms are widely documented for animals and plants, they remain poorly studied in fungi. We used two interfertile species of Epichloë (Clavicipitaceae, Ascomycota), E. typhina and E. clarkii, which are host-specific endophytes of two sympatrically occurring grasses. The life cycle of these obligatory outcrossing fungi entails dispersal of gametes by a fly vector among external fungal structures (stromata). To test for assortative mating, we mimicked the natural fertilization process by applying mixtures of spermatia from both species and examined their reproductive success. Our trials revealed that fertilization is non-random and preferentially takes place between conspecific mating partners, which is indicative of assortative mating. Additionally, the viability of hybrid and non-hybrid ascospore offspring was assessed. Germination rates were lower in E. clarkii than in E. typhina and were reduced in ascospore progeny from treatments with high proportions of heterospecific spermatia. The preferential mating between conspecific genotypes and reduced hybrid viability represent important reproductive barriers that have not been documented before in Epichloë. Insights from fungal systems will deepen our understanding of the evolutionary mechanisms leading to reproductive isolation and speciation.

MateSim: Monte Carlo simulation for the generation of mating tables

In species with sexual reproduction, the mating pattern is an important element for understanding evolutionary and speciation processes. Given a mating pool where individuals can encounter each other randomly, the individual mating preferences define the mating frequencies in the population. However, in every mating process we can distinguish two different steps. First, the encounter between partners, and second, the actual mating once the encounter has occurred. Yet, we cannot always assume that the observed population patterns accurately reflect the individuals' preferences. In some scenarios, individuals may have difficulties to achieve their preferred matings, such as in monogamous species with low population size, where the mating process is similar to a sampling without replacement. In this case, the encounter process will introduce some noise that may disconnect the individual preferences from the obtained mating pattern. Such a difference between mating patterns and mating preferences has previously been shown by different modeling scenarios. Here I present a program that simulates the mating process for both discrete and continuous traits, under different encounter models and individual preference functions, and including effects such as time dependence and aging. The utility of the software is demonstrated by replicating and extending a recent study that showed how patterns of positive assortative mating, or marriage in human societies, may arise from non-assortative individual preferences. The previous result is confirmed and is shown to be caused by the marriage among the "ugliest" and oldest individuals, who after many attempts were finally able to mate among themselves. In fact, I show that the assortative pattern vanishes if an aging process prevents these individuals from mating altogether. The software MateSim is available jointly with the user's manual, at http://acraaj.webs.uvigo.es/MateSim/matesim.htm.