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PubMed2006

Chemically mediated species recognition in closely related Podarcis wall lizards.

In many animals, chemical signals play an important role in species recognition and may contribute to reproductive isolation and speciation. The Iberian lizards of the genus Podarcis, with up to nine currently recognized lineages that are often sympatric, are highly chemosensory and provide an excellent model for the study of chemically mediated species recognition in closely related taxa. In this study, we tested the ability of male and female lizards of two sister species with widely overlapping distribution ranges (Podarcis bocagei and P. hispanica type 1) to discriminate between conspecific and heterospecific mates by using only substrate-borne chemical cues. We scored the number of tongue flicks directed at the paper substrate by each individual in a terrarium previously occupied by a conspecific or a heterospecific lizard of the opposite sex. Results show that males of P. bocagei and P. hispanica type 1 are capable of discriminating chemically between conspecifics and heterospecifics of the opposite sex, but females are not. These results suggest that differences in female, but not male, chemical cues may underlie species recognition and contribute to reproductive isolation in these species. The apparent inability of females to discriminate conspecific from heterospecific males, which is not because of reduced baseline exploration rates, is discussed in the context of sexual selection theory and species discrimination.

Animal Communication↗
PubMed2001

Contrasting patterns of mitochondrial DNA and microsatellite introgressive hybridization between lineages of lake whitefish (Coregonus clupeaformis); relevance for speciation.

We performed a combined analysis of mitochondrial DNA (mtDNA) and microsatellite loci among lake whitefish (Coregonus clupeaformis) populations in order to assess the levels of congruence between both types of markers in defining patterns of genetic structuring, introgressive hybridization and inferring population origins in the hybrid zone of the St. John River basin. A second objective was to test the hypothesis that secondary contact between glacial lineages always resulted in the occurrence of sympatric dwarf and normal whitefish ecotypes. Fish were sampled from 35 populations and polymorphism was screened at mtDNA and six microsatellite loci for a total of 688 and 763 whitefish, respectively. Four lakes harbouring a single whitefish population of normal ecotype admixed with mtDNA haplotypes of different lineages were found. This confirmed that secondary contact between whitefish evolutionary lineages did not always result in the persistence of reproductively isolated ecotypes. Microsatellites further supported the definition of distinct glacial lineages by identifying lineage-specific allelic size groups. They also further supported the hypothesis that ecotypes originated from either a single founding lineage (sympatric divergence) or following secondary contacts between lineages (allopatric divergence), depending on the lake. In general, however, the pattern of population differentiation and introgressive hybridization observed at microsatellites was in sharp contrast with that depicted by mtDNA variation. Both factorial correspondence analysis and analysis of admixture proportion revealed a much more pronounced pattern of introgressive hybridization than depicted by mtDNA analyses. Variable levels of introgression indicated that environmental differences may be as important as the historical contingency of secondary contact in explaining the persistence of sympatric ecotypes and the differential pattern of introgressive hybridization among lakes. Whitefish populations from the St. John River basin hybrid zone represent a rare illustration of a continuum of both morphological and genetic differentiation within a given taxon, spanning from complete introgression to possibly complete reproductive isolation, depending on lakes. Thus, each lake may be viewed as a different temporal snapshot taken throughout the gradual process of speciation.

Animals↗
PubMed2004

Chemical signaling in a wolf spider: a test of ethospecies discrimination.

Chemical signals from female wolf spiders that elicit exploratory behavior and courtship in males are often assumed to be species-specific, but males of some species court in response to silk cues deposited by closely related heterospecific females. Such is the case with the wolf spiders Schizocosa ocreata and S. rovneri, ethospecies reproductively isolated on the basis of differences in behavioral mechanisms during courtship. We explored whether male S. ocreata and S. rovneri reciprocally discriminate species-specific chemical or mechanical cues associated with female silk by using male behavioral response as an assay. Males were exposed to stimulus treatment categories including silk, washed silk, silk extract, and appropriate controls within conspecific or heterospecific female stimulus categories. Male S. ocreata and S. rovneri did not discriminate between conspecific or heterospecific female stimuli, and courtship intensity was greatest on untreated silk. There were no differences in latency to begin courtship or in rates of courtship behaviors attributed to species origin of silk. However, silk treatment (washed silk, extract) had a significant effect on display and exploratory behaviors (e.g., chemoexplore) in both species. Methanol extraction of female silk successfully removed or inactivated a component necessary to elicit active courtship, but extraction did not significantly reduce exploratory behavior, suggesting that a separate compound may be responsible for releasing this behavior. Together, these experiments support the characterization of S. ocreata and S. rovneri as ethospecies reproductively isolated only by female discrimination of species-specific male courtship, and indicate that chemical, but not mechanical cues associated with silk are critical for eliciting male courtship in both species.

Animal Communication↗
PubMed2006

Deleterious epistatic interactions between electron transport system protein-coding loci in the copepod Tigriopus californicus.

The nature of epistatic interactions between genes encoding interacting proteins in hybrid organisms can have important implications for the evolution of postzygotic reproductive isolation and speciation. At this point very little is known about the fitness differences caused by specific closely interacting but evolutionarily divergent proteins in hybrids between populations or species. The intertidal copepod Tigriopus californicus provides an excellent model in which to study such interactions because the species range includes numerous genetically divergent populations that are still capable of being crossed in the laboratory. Here, the effect on fitness due to the interactions of three complex III proteins of the electron transport system in F2 hybrid copepods resulting from crosses of a pair of divergent populations is examined. Significant deviations from Mendelian inheritance are observed for each of the three genes in F2 hybrid adults but not in nauplii (larvae). The two-way interactions between these genes also have a significant impact upon the viability of these hybrid copepods. Dominance appears to play an important role in mediating the interactions between these loci as deviations are caused by heterozygote/homozygote deleterious interactions. These results suggest that the fitness consequences of the interactions of these three complex III-associated genes could influence reproductive isolation in this system.

Animals↗
PubMed2004

Application of Johnson et al.'s speciation threshold model to apparent colonization times of island biotas.

Understanding patterns of diversity can be furthered by analysis of the dynamics of colonization, speciation, and extinction on islands using historical information provided by molecular phylogeography. The land birds of the Lesser Antilles are one of the most thoroughly described regional faunas in this context. In an analysis of colonization times, Ricklefs and Bermingham (2001) found that the cumulative distribution of lineages with respect to increasing time since colonization exhibits a striking change in slope at a genetic distance of about 2% mitochondrial DNA sequence divergence (about one million years). They further showed how this heterogeneity could be explained by either an abrupt increase in colonization rates or a mass extinction event. Cherry et al. (2002), referring to a model developed by Johnson et al. (2000), argued instead that the pattern resulted from a speciation threshold for reproductive isolation of island populations from their continental source populations. Prior to this threshold, genetic divergence is slowed by migration from the source, and species of varying age accumulate at a low genetic distance. After the threshold is reached, source and island populations diverge more rapidly, creating heterogeneity in the distribution of apparent ages of island taxa. We simulated of Johnson et al.'s speciation-threshold model, incorporating genetic divergence at rate k and fixation at rate M of genes that have migrated between the source and the island population. Fixation resets the divergence clock to zero. The speciation-threshold model fits the distribution of divergence times of Lesser Antillean birds well with biologically plausible parameter estimates. Application of the model to the Hawaiian avifauna, which does not exhibit marked heterogeneity of genetic divergence, and the West Indian herpetofauna, which does, required unreasonably high migration-fixation rates, several orders of magnitude greater than the colonization rate. However, the plausibility of the speciation-divergence model for Lesser Antillean birds emphasizes the importance of further investigation of historical biogeography on a regional scale for whole biotas, as well as the migration of genes between populations on long time scales and the achievement of reproductive isolation.

Animals↗
PubMed2001

Spatio-temporal dynamics of the Allonemobius fasciatus- A. socius mosaic hybrid zone: a 14-year perspective.

Long-term studies of hybrid zones can provide valuable insight into a number of questions that have long attracted the attention of evolutionists. These questions range from the stability and fate of hybrid zones to the relative fitness of hybrids. In this paper we report the results of a 14-year survey of the Allonemobius fasciatus-Allonemobius socius hybrid zone. Populations were collected intensively in 1986 and 1987 and then more sporadically through the end of the 1980s and throughout the 1990s. By documenting changes in the genetic composition of populations near and within the zone during this period of time we assessed: the strength of the reproductive isolation between the two species; the relative growth rates (which can be considered a surrogate of relative fitness) of genotype classes corresponding to hybrids and to pure species individuals; and, the power of single-year and multi-year measurements of relative growth rates to predict changes in the genetic composition of mixed populations through time. In brief, we found very large year-to-year variation in the relative growth rates of pure species and hybrid individuals. This variation may reflect the fact that both species are at the edge of their range and perhaps at the limits of their ability to deal with environmental perturbations. As a consequence of the variation, even multi-year estimates of relative growth rates often provided imprecise predictions regarding the future genotypic composition of mixed populations. Despite our limited ability to predict the dynamics of individual populations, some trends are apparent. A. socius, the southern species, has clearly increased in frequency along a transect through the Appalachian Mountains, indicating that the zone is moving north in this region. In contrast, the zone appeared to be more stable along the East Coast transect. Within mixed populations, character-index profiles are often bimodal and stable through time, indicating relatively strong reproductive isolation between the two species that is not being reinforced, nor is it breaking down.

Alleles↗
PubMed2001

Patterns of DNA sequence variation in chromosomally recognized taxa of Anopheles gambiae: evidence from rDNA and single-copy loci.

Patterns of DNA sequence variation in the ribosomal DNA (rDNA) second internal transcribed spacer (ITS2) and five unlinked single-copy nuclear loci were examined for evidence of reproductive isolation among four chromosomally recognized taxa of Anopheles gambiae from West Africa: Savanna, Bamako, Mopti and Forest, as well as sibling species An. arabiensis and An. merus. Included among the single-copy loci were three sequence-tagged random amplified polymorphic DNA (RAPD) loci, two of which (R15 and R37) had been reported as discriminating between Mopti and other chromosomal forms. Each of the five single-copy sequences were highly polymorphic in most samples. However, the R15 and R37 loci had no diagnostic value, and therefore are not recommended as tools in recognition of field-collected An. gambiae chromosomal forms. Although pairwise comparisons between species generally revealed significant levels of differentiation at all five loci, variation was not partitioned by chromosomal form within An. gambiae at any single-copy locus examined. The few exceptions to these trends appear related to a location either inside or nearby chromosomal inversions. At the tryptophan oxygenase locus inside inversion 2Rb, variation was structured only by inversion orientation and not by taxonomic designation even between An. gambiae and An. arabiensis, providing the first molecular evidence that the 2Rb inversion was transferred between species by introgressive hybridization. By contrast, the rDNA showed fixed differences between species and a difference diagnostic for Mopti, consistent with effective, if not complete, reproductive isolation. The apparent disagreement between the data from this locus and multiple single-copy loci within An. gambiae may be explained by the much lower effective population size of rDNA, owing to concerted evolution, which confers increased sensitivity at much shorter divergence times. Taken together with the accompanying reports by della Torre et al. (2001), Favia et al. (2001) and Gentile et al. (2001), our data suggest that neutral molecular markers may not have the sensitivity required to detect isolation between these recently established taxa.

Animals↗
PubMedDate not supplied

Endogamy and inbreeding since the 17th century in past malarial communities in the Province of Cosenza (Calabria, Southern, Italy).

Many authors stressed the importance of considering mating patterns, migration and consanguinity when analysing micro-geographic differences in the distribution of the frequency of genetic traits (thalassaemia and glucose 6 phosphate dehydrogenase deficiency (G6PD) in populations living in areas of past malarial endemy. Therefore, the present work was aimed at estimating the reproductive isolation of Calabria, an Italian region that experienced endemic malaria until very recently. The research was carried out on 15311 records of marriage from Parish Books of four villages located in the past malarial area, and four situated in the non-malarial region. Endogamy rates were high in every village and decreased only in the present century as a consequence of the breakdown of isolation. In the earlier periods, the rates ranged between 93-84% in non-malarial villages, and between 96-66% in the past malarial area. The rate of consanguineous marriages was low in all villages: in the malarial area it was 2.15% on average, whereas in the non-malarial villages it ranged between 2-16%. Its trend increased with time almost everywhere. Concerning values, differences between past malarial and non-malarial villages in earlier periods are not consistent as they ranged from 0.1 x 10(-3) to 1 x 10(-3). In the present century, however, a was higher in the non-malarial villages. Observed changes of the coefficient a since the 19th century are due to the increased frequency of first cousin marriages. Isonymy rates were lower than 2% in all past malarial towns in all periods, whereas in nonmalarial villages they ranged between 1.2-9.5% and increased with time. Inbreeding coefficients F are always higher than alpha values, but show the same trend with time. They were between 0.0006-0.0045 in past malarial towns, and between 0.0017-0.024 in non-malarial villages. In non-malarial villages Fn displayed noticeable negative values in two situations in the earlier periods. In conclusion, given the above mating patterns and the observed distribution of frequencies of G6PD deficient hemizygous and thalassemic heterozygous in the investigated villages, there is clear evidence in this area for the absence of any specific role of reproductive isolation and consanguinity on the distribution of genetic traits related to past malaria presence.

Consanguinity↗
PubMed2002

Host races in plant-feeding insects and their importance in sympatric speciation.

The existence of a continuous array of sympatric biotypes - from polymorphisms, through ecological or host races with increasing reproductive isolation, to good species - can provide strong evidence for a continuous route to sympatric speciation via natural selection. Host races in plant-feeding insects, in particular, have often been used as evidence for the probability of sympatric speciation. Here, we provide verifiable criteria to distinguish host races from other biotypes: in brief, host races are genetically differentiated, sympatric populations of parasites that use different hosts and between which there is appreciable gene flow. We recognize host races as kinds of species that regularly exchange genes with other species at a rate of more than ca. 1% per generation, rather than as fundamentally distinct taxa. Host races provide a convenient, although admittedly somewhat arbitrary intermediate stage along the speciation continuum. They are a heuristic device to aid in evaluating the probability of speciation by natural selection, particularly in sympatry. Speciation is thereby envisaged as having two phases: (i) the evolution of host races from within polymorphic, panmictic populations; and (ii) further reduction of gene flow between host races until the diverging populations can become generally accepted as species. We apply this criterion to 21 putative host race systems. Of these, only three are unambiguously classified as host races, but a further eight are strong candidates that merely lack accurate information on rates of hybridization or gene flow. Thus, over one-half of the cases that we review are probably or certainly host races, under our definition. Our review of the data favours the idea of sympatric speciation via host shift for three major reasons: (i) the evolution of assortative mating as a pleiotropic by-product of adaptation to a new host seems likely, even in cases where mating occurs away from the host; (ii) stable genetic differences in half of the cases attest to the power of natural selection to maintain multilocus polymorphisms with substantial linkage disequilibrium, in spite of probable gene flow; and (iii) this linkage disequilibrium should permit additional host adaptation, leading to further reproductive isolation via pleiotropy, and also provides conditions suitable for adaptive evolution of mate choice (reinforcement) to cause still further reductions in gene flow. Current data are too sparse to rule out a cryptic discontinuity in the apparently stable sympatric route from host-associated polymorphism to host-associated species, but such a hiatus seems unlikely on present evidence. Finally, we discuss applications of an understanding of host races in conservation and in managing adaptation by pests to control strategies, including those involving biological control or transgenic parasite-resistant plants.

Animals↗
PubMed1998

Cryptic speciation and recombination in the aflatoxin-producing fungus Aspergillus flavus.

Aspergillus flavus, like approximately one-third of ascomycete fungi, is thought to be cosmopolitan and clonal because it has uniform asexual morphology. A. flavus produces aflatoxin on nuts, grains, and cotton, and assumptions about its life history are being used to develop strategies for its biological control. We tested the assumptions of clonality and conspecificity in a sample of 31 Australian isolates by assaying restriction site polymorphisms from 11 protein encoding genes and DNA sequences from five of those genes. A. flavus isolates fell into two reproductively isolated clades (groups I and II). The lack of concordance among gene genealogies among isolates in one of the clades (group I) was consistent with a history of recombination. Our analysis included five strains of the closely related industrial fungus A. oryzae, all of which proved to be clonally related to group I.

Aflatoxins↗
PubMed2025

Metschnikowia maris comb. nov., a large-spored yeast species endemic to Serra do Mar Atlantic Rainforest biome, Sao Paulo State, Brazil.

Two yeast isolates from passion flowers were sampled in the southern part of the Serra do Mar Atlantic Rainforest in Sao Paulo State, Brazil. Barcode sequencing and mating experiments showed them to be representatives of Metschnikowia matae var. maris, thus originally named due to the availability of only a single isolate and uncertainties regarding reproductive isolation. The two new isolates being of the complementary mating type to the previously known strain, intravarietal crosses were performed. They yielded a preponderance of two-spored asci, unlike crosses with M. matae var. matae, which led to largely sterile asci. We therefore elevate the variety maris to the rank of species, with the name Metschnikowia maris comb. nov. The holotype is UFMG-CM-Y397T (MATα). Strain UFMG-CM-Y7613A (MAT a) is designated as allotype. The new combination is registered as MB 859665.

Brazil↗
PubMedDate not supplied

Spiking mortality of turkey poults: 1. Experimental reproduction in isolation facilities.

Spiking mortality of turkeys (SMT) is an infectious disease of 5-to-25-day-old turkey poults characterized by acute enteritis and bursal and thymic atrophy. Brooding 1-day-old poults on litter taken from naturally occurring cases successfully reproduced SMT 5 days postexposure. Oral exposure to an organ homogenate made of tissue samples from naturally occurring cases successfully reproduced SMT 5 days postinoculation. Coronaviruses were present in intestinal and bursal contents taken from poults with naturally occurring SMT. They were also present 5 days after exposure in the experimentally reproduced disease. Severe intestinal villus atrophy, bursal follicular lymphoid depletion, and thymic cortical atrophy were present histologically in naturally occurring SMT and in SMT reproduced by either experimental method.

Animals↗
PubMed2006

Gametic isolation in guppies (Poecilia reticulata).

Post-mating reproductive isolating mechanisms may be among the earliest reproductive barriers to emerge among incipient species. Trinidadian guppy, Poecilia reticulata, populations in the Caroni and Oropouche drainages in Northern Trinidad exhibit marked genetic divergence and provide an ideal system in which to search for these barriers. We inseminated virgin females with equal amounts of sperm from two males, a 'native' male from the female's own population and a 'foreign' male from the other drainage. Artificial insemination ensured that mating order and mate choice did not affect the outcome. Paternities were assigned to the resulting broods using microsatellite markers. As predicted, sperm from native males had precedence over foreign sperm. Moreover, this effect was symmetrical for both drainages. In contrast, we detected no native sperm precedence in controls, in which females received sperm from the same and another population within the same drainage. Our results show that gametic isolation can arise between geographically proximate, though genetically divergent, populations of a single species and highlight the potential role of this process in speciation.

Animals↗
PubMed2004

Reproductive versus floral isolation among morphologically similar Serapias L. species (Orchidaceae).

Flowers of the Mediterranean orchid genus Serapias L. form small, dark tubes that vary among taxa in diameter and depth. Visiting insects use the floral tube as shelter and act as pollinators if they touch the sticky viscidium at the rear of the tube and remove the pollinarium. It has been assumed that floral tube size and shape limit access to the flowers and thus may act as a barrier to gene flow between different Serapias species. Here we investigated floral characters and nuclear microsatellite markers in populations belonging to three morphologically similar Serapias species to test whether these species show evidence for floral or reproductive isolation. We found strong overlap of floral traits between two species, suggesting that floral isolation is nonexistent between them. Microsatellite markers applied to the same populations were highly polymorphic and revealed clear genetic differentiation among all three species. These results suggest that reproductive isolation exists, despite the lack of floral isolation between two of the species. In contrast to morphological characters, diagnostic microsatellite alleles were found for all Serapias species. The microsatellite markers could thus provide a useful tool to identify Serapias species and further investigate evolutionary relationships in this fascinating orchid lineage.

Alleles↗
PubMed2004

Geographic isolation of Diplodia scrobiculata and its association with native Pinus radiata.

Diplodia pinea (syn. Sphaeropsis sapinea) is a well-known latent pathogen of Pinus spp. with a worldwide distribution. As such, this fungus is native where pines are endemic in the northern hemisphere and it has been introduced into all countries of the Southern Hemisphere where pines are exotic. The newly described D. scrobiculata (formerly known as the B morphotype of D. pinea) is thought to have a much more limited distribution. D. scrobiculata was first reported as an endophyte and weak pathogen of P. banksiana, where it was found to coexist with D. pinea. Diplodia scrobiculata is now known to have a much broader distribution in Northern America and Europe. In this study, seven Simple Sequence Repeat (SSR) markers were used to evaluate genetic diversity and gene flow between populations of D. scrobiculata. Results indicate a strong geographic isolation between populations of D. scrobiculata from different regions in North America, with unique alleles fixed in the different populations. The data fits the isolation by distance model indicating limited dispersal. Geographic isolation in combination with isolation by distance suggests prolonged reproductive isolation. Intensive collections of endophytes from native P. radiata in California have yielded only D. scrobiculata and not the significantly more pathogenic D. pinea. SSR analysis of three populations of D. scrobiculata from native P. radiata identified many shared alleles among the populations and moderate to high gene flow between them. The three Californian populations are distant and distinct from populations of D. scrobiculata from elsewhere. Under stress conditions, P. radiata is known to be very susceptible to D. pinea in plantations in the Southern Hemisphere. Native P. radiata is currently experiencing severe stress due to pitch canker caused by Fusarium circinatum. Such stress would provide ideal conditions for an associated outbreak of D. pinea. Thus, it is critical to prevent the movement of D. pinea into the last remaining native stands of P. radiata.

Ascomycota↗