Common Morph Research Articles

Flower color polymorphism maintained by overdominant selection in Sisyrinchium sp.

Negative frequency-dependent selection derived from positive frequency-dependent foraging is the best-known selection force maintaining genetic polymorphism within a population. However, in flowering plants, positive frequency-dependent foraging by pollinators is expected to accelerate the loss of low-frequency morphs by conferring a fitness advantage to the common morph, leading to monomorphism. In Japan, a non-native species, Sisyrinchium sp., exhibits conspicuous flower color polymorphism within a population comprising both purple morphs (homozygous recessive) and white morphs (heterozygous or homozygous dominant). Here we quantified genotype-specific reproductive success in order to reveal the contribution of overdominant selection on the maintenance of flower color polymorphism in this species. In artificial pollination experiments using individuals with identified genotypes, female reproductive success was higher in the heterozygote than in either homozygote. The frequency of purple morphs in natural populations (ca. 31%) is similar to the frequency predicted by overdominant selection (25%). Our results suggest that overdominant selection contributes to the maintenance of color morphs in the natural population of this species.

Accelerated evolution of morph-biased genes in pea aphids.

Phenotypic plasticity, the production of alternative phenotypes (or morphs) from the same genotype due to environmental factors, results in some genes being expressed in a morph-biased manner. Theoretically, these morph-biased genes experience relaxed selection, the consequence of which is the buildup of slightly deleterious mutations at these genes. Over time, this is expected to result in increased protein divergence at these genes between species and a signature of relaxed purifying selection within species. Here we test these theoretical expectations using morph-biased genes in the pea aphid, a species that produces multiple morphs via polyphenism. We find that morph-biased genes exhibit faster rates of evolution (in terms of dN/dS) relative to unbiased genes and that divergence generally increases with increasing morph bias. Further, genes with expression biased toward rarer morphs (sexual females and males) show faster rates of evolution than genes expressed in the more common morph (asexual females), demonstrating that the amount of time a gene spends being expressed in a morph is associated with its rate of evolution. And finally, we show that genes expressed in the rarer morphs experience decreased purifying selection relative to unbiased genes, suggesting that it is a relaxation of purifying selection that contributes to their faster rates of evolution. Our results provide an important empirical look at the impact of phenotypic plasticity on gene evolution.

Reproductive biology and colour polymorphism in the food-deceptive Iris lutescens (Iridaceae)

Among the diversity of pollination strategies, food deception is intriguing because of the lack of rewards offered for the pollination services. Floral food deception has been frequently recorded in orchids; however, there is little evidence for food deception outside the Orchidaceae. One putative non-orchid candidate is the Mediterranean Iris lutescens, which displays an heritable polymorphism for flower colour, with yellow and purple morphs. Colour polymorphism is frequently thought to be maintained by negative frequency-dependent selection, where rare morphs are visited more frequently than common ones. Here, we present data on reproductive biology and pollination ecology of the species in the light of the deceptive pollination strategy and the maintenance of flower colour polymorphism. There was no difference in sex allocation and flowering phenology between morphs, with a blooming period early in spring. Both colour morphs are equally self-incompatible and they depend on insects for seed production. The most common visitors with pollen loads are small and large Apoids. These were observed foraging for non-existent nectar (rather than pollen) on Iris flowers. Most of them appear to learn to avoid Iris flowers as classically observed for rewardless species, and there was no evidence of an advantage for the rare morph. Instead, we found that in some populations, large bee pollinators appeared to prefer the most common morph (positive frequency-dependent selection). Iris lutescens appears to be a hitherto neglected but relevant candidate to investigate deceptive reproductive strategy and selective pressures in generalist plants.

First record of the harlequin ladybird, Harmonia axyridis (Coleoptera: Coccinellidae) in the Caucasus

This is the first record of the harlequin ladybird Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) occurring in the Caucasus (Sochi region of Krasnodar territory, Russia). All the adults collected in the field there and reared from collected eggs, larvae and pupae, and their progeny were H. axyridis f. succinea, which is the most common morph in natural populations in South-Eastern Asia and the Russian Far East as well as in invasive populations in the Americas and Europe. In contrast in Western Siberia f. axyridis predominates and this indicates that an occasional introduction from the closest native range in Siberia cannot be considered as the source of the Caucasian population. It is known that populations of H. axyridis can also differ in their photoperiodic responses. The results of earlier experiments on H. axyridis, which originated from the Russian Far East, indicate that the threshold day lengths for the acceleration of preimaginal development and deceleration of reproductive maturation were 13-14 h, while for the invasive populations of this species in Europe these two thresholds are approximately 12 h. In the population studied, the thresholds for both of these photoperiodic responses were also approximately 12 h. Thus, it is concluded that the population of H. axyridis studied is a further eastward continuation of the invasion of this species in Europe.

Differentiation at the MHCIIα and Cath2 loci in sympatric Salvelinus alpinus resource morphs in Lake Thingvallavatn.

Northern freshwater fish may be suitable for the genetic dissection of ecological traits because they invaded new habitats after the last ice age (∼10.000 years ago). Arctic charr (Salvelinus alpinus) colonizing streams and lakes in Iceland gave rise to multiple populations of small benthic morphotypes, often in sympatry with a pelagic morphotype. Earlier studies have revealed significant, but subtle, genetic differentiation between the three most common morphs in Lake Thingvallavatn. We conducted a population genetic screen on four immunological candidate genes Cathelicidin 2 (Cath2), Hepcidin (Hamp), Liver expressed antimicrobial peptide 2a (Leap-2a), and Major Histocompatibility Complex IIα (MHCIIα) and a mitochondrial marker (D-loop) among the three most common Lake Thingvallavatn charr morphs. Significant differences in allele frequencies were found between morphs at the Cath2 and MHCIIα loci. No such signal was detected in the D-loop nor in the other two immunological genes. In Cath2 the small benthic morph deviated from the other two (FST = 0.13), one of the substitutions detected constituting an amino acid replacement polymorphism in the antimicrobial peptide. A more striking difference was found in the MHCIIα. Two haplotypes were very common in the lake, and their frequency differed greatly between the morphotypes (from 22% to 93.5%, FST = 0.67). We then expanded our study by surveying the variation in Cath2 and MHCIIα in 9 Arctic charr populations from around Iceland. The populations varied greatly in terms of allele frequencies at Cath2, but the variation did not correlate with morphotype. At the MHCIIα locus, the variation was nearly identical to the variation in the two benthic morphs of Lake Thingvallavatn. The results are consistent with a scenario where parts of the immune systems have diverged substantially among Arctic charr populations in Iceland, after colonizing the island ∼10.000 years ago.

Morphological variation and sex-biased frequency of wing dimorphism in the pygmy grasshopper Tetrix subulata (Orthoptera: Tetrigidae)

Dimorphism in wing length is well known in many insect species. It is generally believed that a trade-off between dis- persal and reproduction exists, with the long-winged (LW) morph being a better disperser due to its superior flight capability. The short-winged (SW) morph is less mobile and it is hypothesised that females of this morph invest more of their energy reserves in producing offspring. We determined the variation in body and wing size in the pygmy grasshopper Tetrix subulata (Orthoptera: Tetrigidae). The results of the morphological study support the occurrence of two clearly distinct wing morphs in both sexes. SW individuals, especially males, were smaller and in accord with proposed developmental instability showed greater variability in body size than LW individuals. Using data for 700 wild-caught individuals from 10 populations, we demonstrate a variable frequency in the percentage of LW individuals, ranging from all-LW to all-SW populations, even if the LW morph is by far the most common morph in an area. The male-biased percentage of LW individuals recorded in intermediate populations supports a difference in the dispersal reproduction trade-off between the sexes.

Notes on the Flowers and Distribution of the Caribbean Island EndemicIpomoea sphenophylla(Convolvulaceae)

Abstract. The original description of Ipomoea sphenophylla by Urban in 1908, based upon a fruiting specimen, did not include a description of the flowers. The holotype was subsequently lost and plants in flower were recollected in 1994; a neotype was then designated by Howard and McDonald in 1995. An amended description of the species was also supplied, detailing the characteristics of the cotyledons, a swollen root system, and dimorphic flowers. The flowers were described as including a more common morph with a normal funnelform corolla, and an abnormal morph having a 3–5 polypetalous corolla. Presently, both morphs occur together on one sterile individual that is located in the area where the neotype was collected. We have examined approximately 150 additional plants, and all contain funnelform flowers. The polypetalous morphology is therefore atypical of Ipomoea sphenophylla. The species description is emended and the reference to polypetalous flowers is removed.

Colouration of the orb-web spider Gasteracantha cancriformis does not increase its foraging success

The presence of conspicuous colouration in predators is puzzling because natural selection is expected to favour cryptic or disruptive colouration, making predators less detectable by prey. However, the prey attraction hypothesis proposes that conspicuous colour patterns in spiders increase their foraging success by luring prey. Using manipulative experiments we tested the prey attraction hypothesis for the three most common colour morphs of the orb-weaver Gasteracantha cancriformis (yellow, white, and black and white), as well as individuals painted black and individuals painted yellow. Contrary to our predictions, the black painted spiders had the highest number of damaged areas in webs (an indirect measure of foraging success). Black painted spiders were also observed more often consuming prey and with prey remains in the web, although the trend was not significant. Furthermore, there was no difference in the number of prey intercepted by webs of each spider colour and, in the laboratory, Drosophila melanogaster did not choose any of the spider colours preferentially. Our results suggest that colouration in G. cancriformis is costly or neutral in terms of foraging success. Alternatively, we propose that colouration in Gasteracantha could be an aposematic signal.

Survival of Alternative Dorsal-Pattern Morphs in Females of the Anole Norops humilis

Abstract Frequency-dependent predation (FDP), or apostatic selection, is often assumed to be a main evolutionary process to maintain color-pattern polymorphism, particularly when colors are drab. Under this hypothesis, differences in survival rate based on color pattern should be detectable. In many species of anoles, females show variation in dorsal patterns. These patterns consist of drab colors and are thought to have evolved as a predator-avoidance mechanism. We tested the FDP hypothesis for Norops humilis at the La Selva Biological Station in Costa Rica. Three female patterns are found in this population. The dotted pattern occurs in males as well as females, whereas vertebral stripe and reticulated morphs are only found in females. To test FDP, we compared morph frequencies from population data collected at several sampling periods between 1981 and 2007, expecting to see fluctuations in frequencies over time. In addition, we predicted that the most common morph would experience higher mortality. The...

Diurnal changes and frequency dependence in male mating preference for female morphs in the damselfly Ischnura senegalensis (Rambur) (Odonata: Coenagrionidae)

Abstract Ischnura senegalensis females exhibit color dimorphism, consisting of an andromorph and a gynomorph, which might be maintained under a frequency‐dependent process of mating harassment by mate‐searching males. Males change their mating preference for female morph depending on prior copulation experience. Binary choice experiments between two female morphs were carried out in four local populations in the early morning (07.00–09.00 hours) and the afternoon (12.00–14.00 hours), times which mark the onset and the end of diurnal mating activity, respectively. According to the line census along the water's edge, the proportion of andromorphs in the female population varied from 21 to 67% throughout the survey period for four local populations. Males showed non‐biased preference for female morphs in the early morning in each local population, while they chose the common morph in the afternoon. Male mating preference for female morphs was positively correlated to the proportion of female morphs in the population. If the selective mating attacks on the common female morphs inhibit their foraging and/or oviposition behavior, frequency‐dependent male mating attacks might provide a selective force for maintaining the female color dimorphism in I. senegalensis.

Evolution of whole‐body enantiomorphy in the tree snail genus Amphidromus

Diverse animals exhibit left–right asymmetry in development. However, no example of dimorphism for the left–right polarity of development (whole-body enantiomorphy) is known to persist within natural populations. In snails, whole-body enantiomorphs have repeatedly evolved as separate species. Within populations, however, snails are not expected to exhibit enantiomorphy, because of selection against the less common morph resulting from mating disadvantage. Here we present a unique example of evolutionarily stable whole-body enantiomorphy in snails. Our molecular phylogeny of South-east Asian tree snails in the genus Amphidromus indicates that enantiomorphy has likely persisted as the ancestral state over a million generations. Enantiomorphs have continuously coexisted in every population surveyed spanning a period of 10 years. Our results indicate that whole-body enantiomorphy is maintained within populations opposing the rule of directional asymmetry in animals. This study implicates the need for explicit approaches to disclosure of a maintenance mechanism and conservation of the genus.

Disequilibrium in some Cepaea populations

Phenotypic disequilibrium is a measure available, along with morph frequency, in many published sets of sample data recording the polymorphism of shell colour and pattern in the snail Cepaea nemoralis. The relation of disequilibrium to morph frequency for the colour (pink/yellow) and banding (unbanded/banded) loci has been examined for a large and widespread set of data. The direction of disequilibrium is a function of frequency at the two loci in a way that suggests that selection favours combinations of common morphs, whichever they are. This could indicate that such combinations are common because they have been selected. The data are therefore consistent with the proposal that populations are generally subject to selection of varying directions at different places and times, acting on the phenotype. In combination with migration, such selection could lead to prolonged polymorphism.

Reversible frequency-dependent predation of a puffer, Takifugu niphobles (Pisces: Tetraodontidae), related to spatial distribution of colour-polymorphic prey

The puffer Takifugu niphobles is a top predator of hard-shelled prey such as molluscs; its predatory tactics may affect the evolution of prey coloration. Two hypotheses concerning its foraging were tested: (1) T. niphoblesshows frequency dependence in foraging colour-polymorphic prey, and (2) such dependence reverses in response to changes in prey distribution. Captive fish were provided with 70 artificial prey, coloured either dark brown or pale brown, at four frequencies (1 : 4, 2 : 3, 3 : 2, 4 : 1) and in two distribution patterns (uniform and aggregated). When prey were uniformly distributed, frequency and feeding rate significantly influenced colour preference: the common morph was consumed more. When prey were aggregated, frequency significantly affected preference only when the feeding rate was low, in which case the rare morph was consumed more. Thus both hypotheses were supported. The impact of T. niphobles's frequency-dependent predation and its reversal on the colour evolution of prey species, especially molluscs, is discussed. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81, 197–202.

Extreme polymorphism in a Y-linked sexually selected trait.

Males of the livebearing fish, Poecilia parae, exhibit one of the most complex polymorphisms known to occur within populations, whereas females are monomorphic. We describe five distinct male colour morphs and an associated size dimorphism, and demonstrate through pedigree analysis that the locus or loci controlling the male colour polymorphism is linked to the Y-chromosome. Field surveys from 1999 to 2002 of nine populations in Guyana and Suriname, South America, indicate that some morphs are consistently abundant and others are rare, implying that the colour polymorphism has important fitness consequences. By rearing offspring of field-inseminated females, we showed that the common morph is also the most successful morph in terms of reproduction. However, dichotomous choice tests show that two rare morphs are preferred by females over the common morph. These results suggest that alternative male mating strategies, sperm competition, overt male-male competition, or other processes are overriding female preferences in these populations. Furthermore, Y-linkage of the colour polymorphism in P. parae supports the hypothesis that heterogametic sex chromosomes harbour sexually antagonistic traits beneficial to the heterogametic sex.

Reduced reproductive success in small populations of the self‐incompatiblePrimula vulgaris

Summary 1 Habitat fragmentation and the resulting decline in population size can affect biotic interactions and reproductive success of plant species. We investigated the impact of habitat type, population size, morph type and frequency, plant density, floral display and predation on different reproductive components in 16 populations of the distylous self-incompatible perennial herb, Primula vulgaris , a rare, declining species in Belgium. 2 Although habitat type accounted for significant variation in population size, we did not find any relation between habitat type and either reproductive and vegetative characteristics. Population size, however, strongly affected reproductive success, such that plants in small populations produced significantly fewer fruits per plant and seeds per fruit, and therefore fewer seeds per plant. 3 No significant difference was found between morph types for any reproductive characteristic, nor an interaction with population size. However, when morph frequency was strongly biased ( ≥ 1 : 3), the proportion of flowers setting fruit and the number of seeds per fruit were significantly lower in individuals of the common morph type. 4 Within populations, individual plants varied tremendously in size and floral display. Total number of fruits per plant significantly increased with floral display, but the highest fruit set per flower was found at intermediate flower number. 5 The proportion of fruit suffering pre-dispersal predation per plant significantly increased with floral display, but this did not offset the potential fitness gains of producing a large display. Furthermore, the absolute number of predated fruits per plant was significantly and positively affected by the interaction of the total number of fruits per plant and the density of the population.

Floral morph composition and pollen limitation in the seed set of Nymphoides indica populations

We investigated the floral morph composition and seed set of the populations in a heterostylous aquatic plant, Nymphoides indica O. Kuntze (Menyanthaceae). The bias of floral morph ratio in the populations was negatively correlated with the seed set. In the populations with biased floral morph ratio, the pollen on the stigma of the common morphs were dominated by pollen of the same morph, resulting in reduced seed set. The addition of pollen of the opposite morph on the stigmas of the common morph resulted in an increase of fruit and seed set rates. The intermorph pollination rate and the fruit and seed set rates of Nymphoides indica were highest when two floral morphs were close in the populations. Various insect species including Lepidoptera, Diptera and Hymenoptera visited the flowers. However, geitonogamous pollination was common because of the short flying distance of the pollinators. These results indicate that the seed set of Nymphoides indica is limited by the shortage of compatible pollens and that the success of intermorph pollination is influenced by spatial distribution of the two floral morphs. As Nymphoides indica is endangered, conservation of the populations with high fecundity is proposed by us.

Maintenance of a female-limited polymorphism in Ischnura ramburi (Zygoptera: Coenagrionidae)

Colour polymorphisms can be maintained in a population if all morphs have equal fitness on average, if fitness is frequency dependent or if fitness functions cross for some environmental or social variable. We studied female-limited colour polymorphism in the Rambur's forktail damselfly, Ischnura ramburi, in which one female morph looks like the male. The most commonly cited hypotheses to explain this polymorphism involve an advantage to andromorphs of avoiding costly matings through male mimicry. An alternative hypothesis argues that males learn the most common morph and that the polymorphism is maintained by a rare-morph advantage of mating avoidance, irrespective of male mimicry. We tested predictions of the male mimicry hypothesis, learned mate recognition hypothesis (LMR) and two new hypotheses. We used censuses and a mark–resight study to estimate density, sex ratio, morph frequency and mating frequencies. We observed interactions to test for male mimicry and female competition and to evaluate the frequency of mating attempts. Andromorphs were less likely than gynomorphs to receive mating attempts in encounters with males, but did not mate less frequently, or attack males or interrupt oviposition by other females more frequently. Contrary to the LMR hypothesis, the rarer morph was more likely to receive mating attempts. Andromorph frequency was greater in older females than in younger females, suggesting higher mortality or dispersal of gynomorphs. Our results support a modification of the male mimicry hypothesis, the signal detection hypothesis. Together with past studies, our results suggest that the female morphs may be alternative mating avoidance strategies.

Polymorphism, mate choice and sexual selection in the Gouldian finch (Erythrura gouldiae)

Mate choice can result in both assortative mating and directional sexual selection, but few studies have addressed both processes simultaneously. Here we test several hypotheses regarding the possible role of female mate choice in maintaining the face-colour polymorphism of, and affecting directional sexual selection in, the Gouldian finch. These endangered Australian finches are highly sexually dimorphic and are genetically polymorphic for face colour: there are black-, red- and gold-faced individuals. First we showed that Gouldian finches tend to pair positive-assortatively by face colour morph in aviaries. In a laboratory experiment, we tested whether female mate choice is assortative by face colour. Overall, females neither preferred males of the same or of different face colour morphs as themselves. We found weak evidence for positive assortative female choice at one of the two loci involved in determining face colour. Next, we tested whether females showed frequency-dependent mate choice, and found that they preferred neither rare nor common male morphs. In order to test for directional sexual selection on males by female mate choice, we examined the correlations between male morphological traits and attractiveness to females. We found that tail pin length and bill size are correlated with male attractiveness, and may be under sexual selection. Thus, whilst female mate choice may be an important process in determining the evolution of male morphology, and potentially sexual dimorphism, it does not appear to be the primary force behind the assortative mating pattern among the face colour morphs.

Fitness of two phenotypes of Harmonia axyridis (Coleoptera: Coccinellidae)

The coccinellid H. axyridis shows considerable intraspecific variability of elytral patterns. In this paper, we suggest that a genotype that confers a specific pattern of elytra could also confer other specific autecological attributes. The aim of this paper is to compare fitness parameters of two phenotypes (aulica and nigra). Aulica is a common morph characterized by two light red areas that nearly cover the whole elytra, leaving a narrow black border, whereas nigra is completely black; the latter is a rare morph that was naturally obtained from mass cultures. Intraspecific differences occur between H. axyridis phenotypes. The aulica phenotype is more voracious than nigra. Consumption rate of males is higher in aulica than in nigra, but there is no difference for females. Larval biomass of aulica is generally higher than that of nigra. The maximum body-weights reached by larval instars and adults are significantly higher for aulica. Aulica also has greater longevity and reproductive capacity. Only life stage duration of eggs and pre-pupae differ between the two phenotypes. Nigra shows lower fecundity and fertility. Our study showed that the strong differences in phenotype traits of the coccinellid H. axyridis could affect its fitness.

Reversible frequency-dependent switches in male mate choice.

Current sexual-selection theories predict that mating should occur preferentially with the highest-quality partner, and assume that for distinguishing among potential mates the choosy sex applies an internal representation of the characteristics of the desired mate, i.e. a template. Binary choice experiments were performed to test male mate choice between two different female colour morphs in the damselfly Ischnura elegans. Choice experiments were conducted before and after an habituation period, during which males were exposed to only one female colour morph. Given the choice between the two female morphs, males did exhibit a choice for the most recently experienced female morph. This is the first evidence for a reversible switch in mate choice in a frequency-dependent way. In contrast with previous studies on mate choice, template formation in male I. elegans seems not to be based on quality. Switching mate choice in a frequency-dependent manner, choosing the most common morph, probably allows males to minimize their search efforts and to maximize fitness.