Negative assortative mating and maintenance of shell colour polymorphism in Littorina (Neritrema) species

Colour polymorphism is a widespread phenomenon in natural populations of several species. In particular, it is especially common on marine gastropod species from the genus Littorina. Recently, it has been argued that intrapopulation shell colour polymorphism in Littorina fabalis could be caused by negative frequency-dependent sexual selection via a mechanism of mate choice (indirectly estimated via negative assortative mating). Here we try to determine the existence of negative assortative mating in three species of the subgenus Neritrema (L. fabalis, L. obtusata, L. saxatilis) that share a similar shell colour polymorphism, in order to ascertain if this mechanism could represent an ancestral character in this subgenus that could be contributing to the maintenance of the colour polymorphism in each species. Here, we collected or reanalysed from previous studies a sample of mating pairs of the three species from seven locations from NW Spain and NE Russia and estimated assortative mating using the IPSI index. Our results show that all species and populations show a systematic tendency towards negative assortative mating when shell colour is grouped in the broad categories: ‘light’ and ‘dark’. Although, a more detailed analysis of each colour individually suggests that shell colour may not be the main target of assortative mating, but perhaps physically linked to another trait or through pleiotropic effects. This hypothesis opens interesting new lines of research in Littorina snails.

Taxonomy of the New World bee genus Agapostemon Guérin-Méneville – new names and synonymies (Hymenoptera: Halictidae)

Many early taxonomic works on North American bees were published by Europeans using specimens collected in the New World, some with type locations so imprecise that uncertainty on the nomenclatural status remains to this day. Two examples come from Fabricius (1745–1808) who described Andrena virescens Fabricius, 1775 and Apis viridula Fabricius, 1793 from “America” and “Boreal America”, respectively. The former species of Agapostemon Guérin-Méneville, 1844 occurs across most of the United States and southern Canada, the latter presumed an endemic to Cuba. The type materials of these two taxa have never been compared to each other, though a morphology-based phylogenetic analysis placed both in distinct species groups. Here we synonymize Apis viridula under Ag. virescens, thereby making Ag. femoralis (Guérin-Méneville, 1844) available as the name for the Cuban species. A lectotype for Ag. femoralis (the type species for the genus Agapostemon) is hereby designated to stabilize this taxonomy. We also synonymize Ag. obscuratus Cresson, 1869 under Ag. femoralis, suggesting that it represents a dark colour polymorphism. As Ag. cubensis Roberts, 1972 is a junior secondary homonym of Ag. cubensis (Spinola, 1851), we offer Ag. robertsi as a replacement name for the former.

A large and diverse autosomal haplotype is associated with sex-linked colour polymorphism in the guppy

Colour polymorphism provides a tractable trait that can be harnessed to explore the evolution of sexual selection and sexual conflict. Male colour patterns of the Trinidadian guppy (Poecilia reticulata) are governed by both natural and sexual selection, and are typified by extreme pattern colour variation as a result of negative frequency dependent selection. Since guppy colour patterns are often inherited faithfully from fathers to sons, it has been historically presumed that colour genes are physically linked to sex determining loci as a supergene on the sex chromosome. Yet the actual identity and genomic location of the colour pattern genes has remained elusive. We phenotyped and genotyped four guppy Iso-Y lines, where colour was inherited along the patriline, but backcrossed into the stock population every 2 to 3 generations for 40 generations, thereby homogenising the genome at regions unrelated to colour. Using an unbiased phenotyping method to proportion colour pattern differences between and among the Iso-Y lines, we confirmed that the breeding design was successful in producing four distinct colour patterns. Our analysis of genome resequencing data of the four Iso-Y lines uncovered a surprising genetic architecture for colour pattern polymorphism. Genetic differentiation among Iso-Y lines was repeatedly associated with a large and diverse haplotype (~5Mb) on an autosome (LG1), not the sex chromosome (LG12). Moreover, the LG1 haplotype showed elevated linkage disequilibrium and exhibited evidence of sex-specific diversity when we examined whole-genome sequencing data of the natural source population. We hypothesise that colour pattern polymorphism is driven by Y-autosome epistasis, and conclude that predictions of sexual conflict should focus on incorporating the effects of epistasis in understanding complex adaptive architectures.

Shell banding and colour polymorphism of introduced snailCepaea hortensis (Gastropoda, Pulmonata, Helicidae)from some parts of Eastern Europe

The phenotypic composition of C. hortensis colonies was studied in Western Ukraine (more than 30 thousand adults from 23 settlements and 5 administrative regions) and the Moscow region of Russia (more than 500 adults and juveniles from two colonies in Moscow and Vidnoe). Most Western Ukrainian colonies were characterized by the absence of pink shells, the high frequencies of unbanded shells (70% or more), as well as the white ground colour in all banded shells. It is supposed that the latter feature makes it possible to reliably identify the colonies formed by the descendants of the primary introduction of C. hortensis into Western Ukraine, which occurred in the 20th century, from later and independent repeated introductions of this species. Unlike Western Ukraine, snails with yellow banded and pink shells were found in both colonies from the Moscow region. Pink banded shells were registered only in Moscow as well as in two colonies from Lviv. In two of the three colonies mentioned, part of the pink unbanded (Moscow) or all pink shells (Lviv) had a dark lip.

Female preferences for discrete and continuous male colour expression may help reinforce colour polymorphism in a desert lizard

Despite recognition that colour can vary continuously, colour expression in colour polymorphic species is usually treated as discrete. We conducted three experiments to evaluate the extent that discrete and continuous male coloration influenced female mating preferences in long-tailed brush lizards (Urosaurus graciosus). Each experiment provided females with a different social context: a dimorphic choice between a yellow and an orange male (coloration treated as discrete), and a choice between either two orange males or two yellow males (coloration treated as continuous variation). Females preferred orange males over yellow males in the first experiment, and the findings of our second experiment suggested that males with moderate orange coloration were most preferred. In contrast, females behaved randomly with respect to two yellow males. Our findings show that females in colour polymorphic species can evaluate both discrete and continuous aspects of morph coloration during mate assessment, which may help maintain their polymorphism.

Specialist predation covaries with colour polymorphism in tawny owls

Understanding intraspecific phenotypic variation in prey specialisation can help to predict how long-term changes in prey availability affect the viability of these phenotypes and their persistence. Generalists are favoured when the main food resources are unpredictable compared to specialists, which track the availability of the main prey and are more vulnerable to changes in the main food resource. Intraspecific heritable melanin-based colour polymorphism is considered to reflect adaptations to different environments. We studied colour morph-specific diet specialisation in a generalist predator, tawny owl (Strix aluco), during offspring food provisioning in relation to mammal prey density. We hypothesised that the grey morph, with higher fitness than the brown in Northern boreal conditions, is more specialised in mammalian prey than the brown morph, which in turn has higher fitness than the grey in the temperate zone. We found a higher diversity of prey delivered to the nest by brown fathers compared to grey ones, which also depended on the overall mammalian prey availability. Brown fathers provided proportionally fewer mammalian prey than grey in poor, but not in favourable mammal prey years. Our results suggest that the brown morph is more generalistic and reacts more strongly to variations in food supply than the grey morph, which may be a beneficial strategy in an unpredictable environment caused by environmental degradation. Significance statement Diet choice of a species may vary depending on fluctuations in the abundance of their food resource, but also within a population, there can be adaptations to use different food resources. The tawny owl exhibits a grey and a reddish-brown colour morph and is considered a generalist predator eating both mammal and bird prey. We find that the diet of the reddish-brown morph is more diverse than that of the grey. When the tawny owls’ main prey, small mammals, are abundant both colour morphs prey on mammals, but in years with less small mammals, the reddish-brown morph is more prone of switching to small bird predation than the grey. The generalist strategy of the brown morph is likely to be more favourable than a stricter specialisation in small mammals of the grey under recently reoccurring irregularities in small mammal dynamics.