Speciation and gene flow between snails of opposite chirality

Abstract

Left-right asymmetry in snails is intriguing because individuals of opposite chirality are either unable to mate or can only mate with difficulty, so could be reproductively isolated from each other. We have therefore investigated chiral evolution in the Japanese land snail genus Euhadra to understand whether changes in chirality have promoted speciation. In particular, we aimed to understand the effect of the maternal inheritance of chirality on reproductive isolation and gene flow. We found that the mitochondrial DNA phylogeny of Euhadra is consistent with a single, relatively ancient evolution of sinistral species and suggests either recent "single-gene speciation" or gene flow between chiral morphs that are unable to mate. To clarify the conditions under which new chiral morphs might evolve and whether single-gene speciation can occur, we developed a mathematical model that is relevant to any maternal-effect gene. The model shows that reproductive character displacement can promote the evolution of new chiral morphs, tending to counteract the positive frequency-dependent selection that would otherwise drive the more common chiral morph to fixation. This therefore suggests a general mechanism as to how chiral variation arises in snails. In populations that contain both chiral morphs, two different situations are then possible. In the first, gene flow is substantial between morphs even without interchiral mating, because of the maternal inheritance of chirality. In the second, reproductive isolation is possible but unstable, and will also lead to gene flow if intrachiral matings occasionally produce offspring with the opposite chirality. Together, the results imply that speciation by chiral reversal is only meaningful in the context of a complex biogeographical process, and so must usually involve other factors. In order to understand the roles of reproductive character displacement and gene flow in the chiral evolution of Euhadra, it will be necessary to investigate populations in which both chiral morphs coexist.

Figure 1. Sinistral Species

(A) E. quaesita from Sendai and (B) E. murayamai from Myojo-san (in cave); dextral (C) E. senck. senckenbergiana from Imajyo, (D) E. senck. amoriensis from Tamayama, (E) E. senck. amoriensis from Iide-san, and (F) E. senck. ibukicola from Mt. Fujiwara. [Photos: AD and SC]

Figure 2. Distributions of Four Euhadra Species on the Main Japanese Island of Honshu

E. senck. aomoriensis specimens that were used for DNA analysis were collected from Tamayama, Tsugaru, and Iide-san. E. murayamai is confined to a single mountain (Myojo-san). The remaining marked sites refer to sites of snails in Figure 1, as well as to others that are specifically referred to in the main text. Sinistral species are shown in red and dextral species in blue.

Figure 3. Neighbour-Joining Phylogeny

16S rRNA rate-corrected neighbour-joining phylogeny, rooted using Nesiohelix bipyramidalis, showing the relationship between mitochondrial DNA lineages from dextral and sinistral Euhadra. The most parsimonious explanation is a single, relatively ancient evolution of sinistral snails. Lineages within the box are shown in detail in Figure 4. Bootstrap support of more than 70% is shown below the node. Shape parameter = 0.30.

Figure 4. Neighbour-Joining Phylogeny Subtree

16S rRNA rate-corrected neighbour-joining phylogeny (subtree from Figure 3) showing the relationship between E. quaesita, E. senck. aomoriensis, and E. murayamai mitochondrial lineages. All unlabelled tips are E. quaesita. Both E. senck. aomoriensis and E. murayamai are polyphyletic. The polyphyly of E. senck. aomoriensis can be explained by either single-gene speciation or mitochondrial introgression. Bootstrap support shown for important nodes only.

Figure 5. Maximum Parsimony Tree of Two Characters

The tree is based on (A) genital characters and (B) shell characters. The number after each species is the sample location (see also Tables S1–S3). Sinistral species are shown in red and dextral species in blue.

Figure 6. Proportion of Sinistral Snails Amongst Offspring of Sinistrals Compared with Frequency of Sinistrals in the Population

The curves are the boundaries defined by extreme values of α, the parameter that describes the degree of interchiral mating (lower curve: random mating between chiral morphs, α = 0; upper curve: no interchiral mating, α = 1). The space between the two curves represents intermediate values of α. The intercepts illustrate the predictive nature of the model; e.g., since interchiral mating is not possible in Euhadra (α = 1), then if a mixed population of Euhadra contains 40% sinistrals, the model predicts that 80% of the offspring of sinistral snails should be sinistral. For a species with no barriers to interchiral mating (α = 0), the model predicts that 65% of the offspring should be sinistral. Finally, if the mixed population is actually two separate species that might have formed by single-gene speciation, then the prediction is that sinistral snails should always give birth to sinistral offspring (y = 1, indicated by the dotted line; this is the second, unstable equilibrium that is referred to in the main text). Equilibria are calculated from Equation 5 in Protocol S1.

Figure 7. Proportion of Sinistral Snails Amongst Offspring of Sinistrals Compared with Frequency of Sinistrals in the Population, with Reduced Mating Frequency or Fertility for the Rare Chiral Morph

We assumed a strong frequency-dependent mating disadvantage against the rare morph, no interchiral mating (α = 1) and equal sexual selection on the two sexes. The graph shows eight runs that start close to the border separating two alternative outcomes, which runs approximately vertically down the middle of x-axis of the graph (when the frequency of sinistrals is about 0.5). As there was no interchiral mating, then if the starting condition is two distinct populations (sinistral snails have only sinistral alleles and vice versa), then the two chiral morphs stay separate: the points along the top horizontal line represent this, indicating competition between two reproductively isolated types, or single-gene speciation (y = 1; as in Figure 6, this is also the second, unstable equilibrium that is referred to in the main text). In all other circumstances, the model shows that the population moves to equilibrium, so that there is extensive gene flow between different chiral morphs. For further information, see Results or section 3.i in Protocol S1.

Figure 8. Map of the Periglacial Region in Japan at the Height of the Last Glaciation

The last glaciation peaked about 20,000 years ago. Most of northern Tohoku must have been unvegetated (white shading), so Euhadra probably colonised from the south in the early Holocene. Brown shading shows regions above sea level, blue is sea, and the black outline indicates present-day Japan. The distribution of the periglacial region in earlier epochs is more uncertain. Data compiled from [–58].