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PubMed2006

An analysis of species boundaries and biogeographic patterns in a cryptic species complex: the rotifer--Brachionus plicatilis.

Since the advent of molecular phylogenetics, there is increasing evidence that many small aquatic and marine invertebrates--once believed to be single, cosmopolitan species--are in fact cryptic species complexes. Although the application of the biological species concept is central to the identification of species boundaries in these cryptic complexes, tests of reproductive isolation do not frequently accompany phylogenetic studies. Because different species concepts generally identify different boundaries in cryptic complexes, studies that apply multiple species concepts are needed to gain a more detailed understanding of patterns of diversification in these taxa. Here we explore different methods of empirically delimiting species boundaries in the salt water rotifer Brachionus plicatilis by comparing reproductive data (i.e., the traditional biological species concept) to phylogenetic data (the genealogical species concept). Based on a high degree of molecular sequence divergence and largely concordant genetic patterns in COI and ITS1, the genealogical species hypothesis indicates the existence of at least 14 species--the highest estimate for the group thus far. A test of the genealogical species concept with biological crosses shows a fairly high level of concordance, depending on the degree of reproductive success used to draw boundaries. The convergence of species concepts in this group suggests that many of the species within the group may be old. Although the diversity of the group is higher than previously understood, geographic distributions remain broad. Efficient passive dispersal has resulted in global distributions for many species with some evidence of isolation by distance over large geographic scales. These patterns concur with expectations that micro-meiofauna (0.1-1mm) have biogeographies intermediate to microbial organisms and large vertebrates. Sympatry of genetically distant strains is common.

Animals↗
PubMed2002

Phylogeographic analysis of the brooding brittle star Amphipholis squamata (Echinodermata) along the coast of New Zealand reveals high cryptic genetic variation and cryptic dispersal potential.

Direct development in benthic marine invertebrates is usually associated with narrow geographical range, low rates of colonization, and low levels of gene flow. Paradoxically, the small brittle star Amphipholis squamata broods its larvae to a crawl-away juvenile stage, yet has a cosmopolitan distribution. Using sequence and restriction-fragment-length-polymorphisms (RFLP) analyses of nuclear and mitochondrial DNA from 16 coastal populations throughout New Zealand, we tested whether the species is indeed a poor disperser, as may be expected from its brooding habit. We predicted that local and regional populations would be genetically structured according to isolation by distance. We also suspected that this ubiquitous "species" is composed of a variety of cryptic taxa in different geographic areas, as has been discovered in an increasing number of marine invertebrates. We found evidence of four genetically divergent and reproductively isolated lineages that can exist in syntopy. Lineages vary in abundance, haplotype diversity, and geographic distribution. The partitioning of genetic variation within the most common lineage, as well as the geographic distribution of the four lineages, suggest a north/south split. This pattern is consistent with known New Zealand marine biogeographic zones and appears to be linked to the regime of oceanic circulation, which is characterized by subtropical, southward-moving water masses in the north, and sub-Antarctic, northward-moving water in the south. We conclude that the dispersal ability of A. squamata is regionally restricted but with sporadic long-distance dispersal, which serves to increase local genetic variation. Our results support the idea that dispersal occurs through passive transport by drifting or rafting on macroalgae, which A. squamata commonly inhabits, and emphasize that poor dispersal ability is not necessarily a corollary of direct development.

Analysis of Variance↗
PubMed2002

Assortative fertilization and selection at larval stage in the mussels Mytilus edulis and M. galloprovincialis.

Assortative mating (prezygotic isolation) and reduced hybrid fitness (postzygotic isolation) are typically invoked to explain the stability of hybrid zones. In the tension zone model, these factors work in opposition to migration, which promotes genetic homogeneity. Many marine animals migrate over long distances through a planktonic larval stage. Therefore, strong reproductive isolation is needed to maintain stable marine hybrid zones. However, surprisingly little is known about mating preferences and hybrid fitness in marine organisms. Smooth-shelled mussels (Mytilus spp.) form a well-known species complex, with hybridization over extensive areas such as the contact zone of M. edulis and M. galloprovincialis around European Atlantic coasts. This paper reports direct experimental evidence of assortative fertilization, hybrid larval inviability, and early heterosis for growth rate in M. edulis and M. galloprovincialis. Four crosses between pure M. edulis and M. galloprovincialis were analyzed with a new polymerase-chain-reaction-based diagnostic marker. Gamete competition between taxa was allowed in two out of the four crosses. Genotype frequencies observed at an early stage (36 h after fertilization) unambiguously revealed assortative fertilization when gamete competition was allowed. A significant reduction in hybrid viability was subsequently observed during the larval stage. At the same stage an antagonistic effect, heterosis, was observed on growth rate. However, even if heterosis is observed in the F1, it is expected to vanish in subsequent hybrid generations. Although specialization for different habitats and asynchronous spawning have been mentioned as factors contributing to the maintenance of the blue mussel hybrid zone in Europe, we argue that assortative fertilization and reduced hybrid fitness are important factors that also contribute to the stabilization of this zone. These results emphasize that multiple factors may act concomitantly in a barrier to gene flow, especially in complex life cycles. Furthermore, they show that assortative mating through gamete preference, as already demonstrated for sea urchins, may play a role in speciation processes taking place in the sea.

Animals↗
PubMed2007

Characterization and distribution of Pogonomyrmex harvester ant lineages with genetic caste determination.

Genetic caste determination has been described in two populations of Pogonomyrmex harvester ants, each comprising a pair of interbreeding lineages. Queens mate with males of their own and of the alternate lineage and produce two types of diploid offspring, those fertilized by males of the queens' lineage which develop into queens and those fertilized by males of the other lineage which develop into workers. Each of the lineages has been shown to be itself of hybrid origin between the species Pogonomyrmex barbatus and Pogonomyrmex rugosus, which both have typical, environmentally determined caste differentiation. In a large scale genetic survey across 35 sites in Arizona, New Mexico and Texas, we found that genetic caste determination associated with pairs of interbreeding lineages occurred frequently (in 26 out of the 35 sites). Overall, we identified eight lineages with genetic caste determination that always co-occurred in the same complementary lineage pairs. Three of the four lineage pairs appear to have a common origin while their relationship with the fourth remains unclear. The level of genetic differentiation among these eight lineages was significantly higher than the differentiation between P. rugosus and P. barbatus, which questions the appropriate taxonomic status of these genetic lineages. In addition to being genetically isolated from one another, all lineages with genetic caste determination were genetically distinct from P. rugosus and P. barbatus, even when colonies of interbreeding lineages co-occurred with colonies of either putative parent at the same site. Such nearly complete reproductive isolation between the lineages and the species with environmental caste determination might prevent the genetic caste determination system to be swept away by gene flow.

Animals↗
PubMed2005

Sex pheromones of five olethreutine species (Lepidoptera: Tortricidae) associated with the seedlings and fruits of mangrove plants in the Ryukyu Islands, Japan: identification and field evaluation.

The sex pheromones of three Cryptophlebia, one Centroxena, and one Eucosma species (Lepidoptera: Olethreutinae) inhabiting mangroves in the Ryukyu Islands, Japan, were studied with coupled gas chromatography-electroantennographic detection, and GC-mass spectrometry (GC-MS). The larvae of each Cryptophlebia species are specifically associated with viviparous seedlings from one of three mangrove Rhizophoraceae plants. Whereas three EAG-active alcohol components, (Z)-8-dodecen-1-ol, (E)-8-dodecen1-ol, and dodecan-1-ol, in a ratio of 100:12:4, were identified from the pheromone gland extract of female of C. horii (host: Bruguiera gymnorrhiza), two other sibling species produced the corresponding acetates, i.e., (Z)-8-dodecenyl acetate, (E)-8-dodecenyl acetate, and dodecyl acetate, in a 100:2:3 ratio from Cryptophlebia palustris (host: Rhizophora stylosa in Iriomote-jima Island) and in a 100:7:13 ratio from C. amamiana (host: Kandelia candel in Amami-oshima Island). The double bond positions of the monounsaturated components were confirmed by GC-MS analyses of their adducts with dimethyl disulfide. On the other hand, the larvae of Centroxena sp. feed on fruits of Sonneratia alba, another mangrove plant in the Sonneratiaceae, and the extract of the female pheromone glands contained (8E,10E)-dodecadienyl acetate and dodecyl acetate in a ratio of 100:5. The double bond position of the diunsaturated compound was confirmed by GC-MS analysis of its adduct with 4-methyl-1,2,4-triazoline-3,5-dione. (E)-9-Dodecenyl acetate w sively identified in the pheromone gland extract of Eucosma coniogramma females reared from seedlings of B. gymnorrhiza. Although the roles of minor components have not been revealed by field tests, synthetic lures baited with the main pheromone component of each species successfully attracted the target males, confirming that the sex pheromone is one of the most important factors for their reproductive isolation.

Animals↗
PubMed2000

Genetic differentiation in Tetranychus urticae (Acari: Tetranychidae): polymorphism, host races or sibling species?

Based on allozyme electrophoresis at the Pgm locus and nuclear ribosomal DNA (ITS2) sequences, we studied the genetic variation of the two-spotted spider mite Tetranychus urticae Koch collected on rose bay, Nerium oleander L. (Apocynaceae), from several localities around the Mediterranean basin. In addition, we compared these results with those of Navajas et al. (1998) and Tsagkarakou (1997) who collected from several other host plants from the Mediterranean. In the western part of this area (Spain, France, Tunisia), we found the individuals collected from rose bay to be clearly genetically differentiated from other samples. No evidence of such host-associated differentiation was detected in the eastern Mediterranean (Italy and Greece). The genetic differentiation of mites collected on rose bay was investigated further by studying the reproductive incompatibilities between populations in Greece and in France and a laboratory strain reared on bean, Phaseolus vulgaris, in France. Reciprocal crosses performed between these strains revealed variable levels of incompatibility, spanning from partial to complete reproductive isolation. In all cases incompatibility was asymmetric. We designed a test based on double-mating to establish the fertilization status of females in fully incompatible crosses. These crosses showed that the females had been inseminated, which suggests that the barrier to reproduction is not of a prezygotic behavioral nature. The data raises the question of the relative role of ecological factors (host plant) and geographical distance, in the ongoing differentiation process potentially leading to speciation.

Alleles↗
PubMed2001

DNA analysis of transferred sperm reveals significant levels of gene flow between molecular forms of Anopheles gambiae.

Anopheles gambiae populations in west Africa are complex, being composed of multiple, sympatric subpopulations. Recent studies have failed to reveal significant genetic differences among subpopulations, stimulating a debate regarding the levels of gene flow among them. The observed homogeneity may be the consequence of substantial contemporary gene flow or it may be that reproductive isolation is complete, but too recent for the accumulation of significant levels of genic divergence. Here, we report the results of a study estimating contemporary levels of gene flow between An. gambiae subpopulations by analysing females and transferred sperm removed from their reproductive systems. A total of 251 female and associated sperm extracts was analysed from a single site in Mali. Two molecular forms of An. gambiae, the M- and S-forms, occurred in sympatry at this site. Overall, we found very strong positive assortative mating within forms, however, we did observe significant hybridization between forms. In the M subpopulation 2/195 females (1.03%) contained sperm from S-form males and in 55 S-form females we found one female containing M-form sperm (1.82%). We also identified a mated M xS hybrid adult female. From mating frequencies, we estimate the Nem between the M- and S-form at 16.8, and from the adult hybrid frequency at 5.6. These values are consistent with our earlier estimate, based on FST for 21 microsatellite loci in which Nem = 5.8. We conclude that the general lack of genetic divergence between the M and S subpopulations of An. gambiae can be explained entirely by contemporary gene flow.

Animals↗
PubMed2006

The genetic basis of floral traits associated with mating system evolution in Leptosiphon (Polemoniaceae): an analysis of quantitative trait loci.

Mapping of quantitative trait loci (QTL) was used to investigate the genetic architecture of divergence in floral characters associated with the mating system, an important adaptive trait in angiosperms. Two species of Leptosiphon (Polemoniaceae), one strongly self-fertilizing (L. bicolor) and the other partially outcrossing (L. jepsonii), were crossed to produce F2 and both backcross progenies. For each crossing population, a linkage map was created using amplified fragment length polymorphism markers, and QTL were identified for several dimensions of floral size. For each of the five traits examined, three to seven QTL were detected, with independent datasets yielding congruent results in some but not all cases. The phenotypic effect of individual QTL was generally moderate. We estimated that many of the QTL were additive or showed dominance toward L. bicolor, whereas comparison of mean trait values for parental and cross progenies showed apparent overall dominance of L. jepsonii traits. Colocalization of QTL for different dimensions of floral size was consistent with high phenotypic correlations between floral traits. Substantial segregation distortion was observed in marker loci, the majority favoring alleles from the large-flowered parent. A low frequency of male sterility in the F2 population is consistent with the Dobzhansky-Muller model for the evolution of reproductive isolation.

Biological Evolution↗
PubMedDate not supplied

Counter current transfer and back transport of 3H-PGF2 alpha in the cow's broad ligament vasculature ipsilateral and contralateral to the corpus luteum.

Eight cows of similar age (5-7 years) were chosen for the experiment. Isolated reproductive tract was supplied with autologous oxygenated and heated (40 degrees C) blood through the uterine artery and ovarian artery. 3H-PGF2 alpha in total dose of 2 MBq (10(7) cpm) was injected into each of the uterine lumen of isolated organ. Blood samples were collected at 5 min intervals during 120 min of experiment using cannulae inserted into the branches of uterine arteries about 1 cm below the horns and from ovarian arteries inserted 0.5 cm below the ovaries. The concentration of 3H-PGF2 alpha found in blood plasma taken from uterine artery or from ovarian artery on the side with active corpus luteum (CL) was significantly lower (p less than 0.001) compare with contralateral side to active CL. Radioactive PGF2 alpha found in branches of uterine arteries on both ipsilateral and contralateral side to CL was significantly higher (p less than 0.001) compare to ovarian artery of the same side. It is concluded that absorption of 3H-PGF2 alpha from uterine lumen into venous blood as well as its counter current transfer in area of broad ligament vasculature were reduced on the side of uterine horn with active CL probably as an effect of estrogen:progesterone ratio on vascular constriction in area of uterine vasculature.

Adnexa Uteri↗
PubMed2005

Ribosomal DNA sequence variation among sympatric strains of the Cyclotella meneghiniana complex (Bacillariophyceae) reveals cryptic diversity.

Cyclotella meneghiniana Kützing is one of the most commonly found and intensively studied freshwater diatom species. However, it is considered taxonomically problematic because of its unusually wide ecological range and large frustule ultrastructural variation. As part of a study of morphological and genetic variation in this morphospecies, we surveyed nucleotide variation in the hypervariable D1/D2 regions of the 28S rDNA, in the ribosomal internal transcribed spacer region (containing ITS1, the 5.8S rDNA and ITS2) and in the 18S rDNA in a collection of 20 sympatric strains. High genetic variability and strong indications of genetic structure among the Cyclotella meneghiniana strains were found. Representatives of four genetically distinct--apparently reproductively isolated--groups were revealed among them. The random distribution of ITS variation within these four groups indicated that the genetic structure in Cyclotella meneghiniana can probably be explained by the presence of cryptic sexual species rather than by the lack of allogamous sexual reproduction. The morphological features traditionally used for species identification in this group cannot distinguish these putative cryptic species.

Animals↗
PubMed2001

Incongruence of mitochondrial and nuclear gene trees in the Carabid beetles Ohomopterus.

We studied the molecular phylogeny of the carabid subgenus Ohomopterus (genus Carabus), using two mitochondrial (mt) DNA regions (16SrRNA and NADH dehydrogenase subunit 5) and three nuclear DNA regions (wingless, phosphoenolpyruvate carboxykinase, and an anonymous locus). We revisited the previously reported incongruence between the distribution of mtDNA markers and morphologically defined species (Su et al., 1996; J. Mol. Evol. 43:662-671), which those authors attributed to "type switching", a concerted change in many morphological characters that results in the repeated evolution of a particular morphological type. Our mtDNA gene tree obtained from 44 individuals representing all 15 currently recognized species of Ohomopterus revealed that haplotypes isolated from individuals of a single "species" were frequently separated into distant clades, confirming the previous report. The three nuclear markers generally conformed better-with the morphologically defined species than did the mitochondrial markers. The phylogenetic signal in mtDNA and nuclear DNA data differed strongly, and these two partitions were significantly incongruent with each other according to the incongruence length difference test of Farris et al. (1994; Cladistics 10:315-320), although the three nuclear partitions were not homogeneous either. Our results did not support the type-switching hypothesis that had been proposed to fit the morphological data to the mitochondrial gene tree: The incongruence of the mtDNA tree with other nuclear markers indicates that the mtDNA-based tree does not reflect species history any better than the morphological data do. Incongruence of gene trees in Ohomopterus may have been promoted by the complex processes of geographic isolation and hybridization in the Japanese Archipelago that have led to occasional gene flow and recombination between separated entities. The occurrence of reticulate patterns in this group is intriguing, because species of Ohomopterus exhibit extremely divergent genitalic structures that represent a highly efficient reproductive isolation mechanism.

Animals↗
PubMed1994

Cytoplasmic incompatibility. Giant steps sideways.

The horizontal transfer of a bacterial endosymbiont that is intimately associated with reproductive isolation in insects is now feasible and may, in principle, lead to new strategies for biological pest control.

Animals↗
PubMed1996

Natural selection and random genetic drift as causes of evolution on islands.

The evolutionary processes responsible for adaptation and speciation on islands differ in several ways from those on the mainland. Most attention has been given to the random genetic drift that arises when a population is founded from just a few colonizing genomes. Theoretical obstacles to "founder effect speciation' are discussed, together with recent proposals for avoiding them. It is argued that although certain kinds of epistasis can facilitate the evolution of strong reproductive isolation, this favours divergence by selection as much as by random drift.

Alleles↗
PubMed1994

Postcopulatory, prezygotic isolation: intraspecific and interspecific sperm precedence in Tribolium spp., flour beetles.

We investigated intraspecific and interspecific patterns of paternity in Tribolium castaneum, the flour beetle, by mating females either to pairs of conspecific males or to one conspecific and one heterospecific male of the closely related species, T. freemani. Females of both species store sperm in the spermatheca after copulation and postcopulatory, prezygotic reproductive isolation has been reported between this pair of species. When conspecific males of contrasting genotype are mated simultaneously to T. castaneum females, we observed extremely high levels of variation among females in the pattern of sperm precedence as shown by the offspring genotypes. In contrast, T. castaneum females mated simultaneously to a conspecific and a heterospecific male produced over 99 per cent conspecific progeny. When conspecific males were mated sequentially to T. castaneum females, within 3-7 days, all offspring were sired by the second male. In contrast, when a conspecific male was replaced with a T. freemani male, most females continued to produce only first-male, conspecific offspring for the next 10 days. When a heterospecific male was replaced with a conspecific male, T. castaneum females changed from producing hybrid to conspecific offspring within 3 days. We discuss the evolutionary implications of these findings.

Animals↗
PubMed2003

Developmental stability and adaptive variability of male genitalia in sexually dimorphic beetles.

Animal genitalia often show distinct developmental and evolutionary relationships with other parts of the body. Morphological observations of 29 sexually dimorphic and monomorphic beetle species in 16 genera of families Scarabaeidae and Lucanidae, Coleoptera, in 53 locations revealed that male genitalia size was consistently and distinctly less variable than that of other body parts within the same population, while it differentiated more readily among different populations than other body parts. The most noticeable genitalia size differentiation occurred in populations that coexisted with morphologically and ecologically similar congeneric species. Such differentiation may indicate selection for reproductive isolation. These characteristics of genitalia morphology may have been instrumental in generating the speciation pattern seen in most beetles.

Animals↗
PubMed2006

Replicated population divergence caused by localized coevolution? A test of three hypotheses in the red crossbill-lodgepole pine system.

Several lines of evidence support the hypothesis that local populations of red crossbills (Loxia curvirostra complex) enter into a predator-prey arms race with lodgepole pine (Pinus contorta latifolia) in the absence of competing pine squirrels (Tamiasciurus hudsonicus). Nevertheless, the alternative hypotheses that neutral evolution or factors other than squirrels have caused crossbill population differentiation have not been thoroughly tested. We compared crossbill and pine cone morphology between island populations where squirrels are absent or present, and mainland sites where squirrels are present, in order to distinguish among these hypotheses. All comparisons supported an effect of squirrel absence, not island status, on crossbill and cone morphology. Hence our results provide further evidence that strong localized coevolutionary interactions in a geographic mosaic have driven adaptive population differentiation. In addition, vocal differentiation of crossbills was related to the absence of squirrels, but not to island status. As morphological and vocal differentiation is correlated with reproductive isolation in crossbills, the geographic mosaic of coevolution also seems to promote ecological speciation.

Adaptation, Physiological↗
PubMed2001

Incipient sexual isolation in the nasuta-albomicans complex of Drosophila: no-choice experiments.

Drosophila nasuta nasuta and Drosophila nasuta albomicans are cross-fertile races of Drosophila. Hybridization between these races in the laboratory has given rise to new races (Cytoraces), among which karyotypic composition differs from one another and also from those of the parental races. In this study, we search for the evidence of incipient reproductive isolation among the parental races and four Cytoraces by assessing the fraction of no-matings, mating latency and copulation duration in all possible types of homo- and heterogamic crosses (N = 4184). In no-choice conditions, the latency time (time to initiation of copulation) is lower in homogamic crosses than in heterogamic crosses for both parental races and Cytoraces. Latency time and copulation duration are negatively correlated, whereas fraction of no matings is positively correlated with latency time. Thus these six closely related races of the nasuta-albomicans complex show the initiation of the earliest stages of pre-zygotic isolation, manifested as a tendency for matings to be initiated earlier and more often, and for a longer duration, among homogamic rather than heterogamic individuals

Animals↗
PubMed1995

Wolbachia and cytoplasmic incompatibility in mycophagous Drosophila and their relatives.

Bacterial symbionts belonging to the genus Wolbachia are associated with postzygotic reproductive incompatibility in a number of insect species. Using polymerase chain reaction (PCR) amplification of bacterial gene sequences, strains from 10 species belonging to the closely related quinaria, testacea and tripunctata groups of Drosophila were screened for the presence of Wolbachia in their reproductive tissues. Those screened included the mycophagous species D. falleni, D. recens, D. phalerata, D. testacea, D. neotestacea, D. orientacea, D. putrida and D. tripunctata, and the nonmycophagous species D. palustris and D. quinaria. Two species, D. recens and D. orientacea, were found to be infected with Wolbachia. Subsequent tests of four additional strains of D. recens found all to be infected with the bacteria. It was established that these bacteria cause partial cytoplasmic incompatibility in D. recens by antibiotic curing followed by crosses between cured and uncured strains. Curing was confirmed by a PCR assay. Although most species of insects shown to be infected with Wolbachia are cosmopolitan and/or have undergone recent range expansion in association with human activity, D. recens and D. orientacea are endemic species with specialized ecological habits. Preliminary molecular phylogenetic analysis indicates that, among the species we examined, D. quinaria is most closely related to D. recens. To determine whether the bacteria are involved in reproductive isolation between these two species, reciprocal crosses were carried out between D. quinaria and both infected and uninfected (cured) strains of D. recens. Although these species did mate with each other, all interspecific crosses failed to yield hybrid progeny, indicating that the bacteria are not responsible for reproductive incompatibility between these species.

Animals↗