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PubMed1992

Host specificity of Laelaps (Acari: Laelapidae) in central Brazil.

A study of associations between small mammals and ectoparasites in two adjacent nature reserves near Brasilia, Federal District, Brazil, revealed that ectoparasitic mites of the genus Laelaps Koch are host specific. Data on the prevalence and intensity of mite infestations were combined with measures of the reproductive activity of mite populations to estimate the association of each host-mite pair. Twelve morphologically distinct species of Laelaps occurred in monoxenous and oligoxenous associations with the cricetid rodent genera Oryzomys, Nectomys, Rhipidomys, and Calomys. Nine were referred to species recognized at the time of the study; three were described as new species in a recent publication. When mites initially assigned as the same species infested more than one host species, an examination of the morphological variation within and among mite specimens supported the hypothesis that mites infesting different hosts were reproductively isolated populations. Dispersal pathways apparently were restricted, occurring primarily between conspecific individuals. In one instance, two morphologically similar groups of Laelaps paulistanensis Fonseca were associated with two closely related rodent species, Oryzomys fornesi Massoia and O. nigripes (Olfers). Although the data presented here indicate that these two groups of mites are conspecific, further study is warranted.

Animals↗
PubMed1996

Population genetics of Ixodes scapularis (Acari: Ixodidae) based on mitochondrial 16S and 12S genes.

Blacklegged tick, Ixodes scapularis Say, is the principal vector of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner in the eastern half of the United States. Populations exhibit extreme variation in morphology, host usage, development time, and behavior. We examined sequence variation in the 16S and 12S mitochondrial ribosomal DNA genes to determine genetic relationships among I. scapularis collections from throughout its range. Single strand conformation polymorphism analysis of 300 bp of the 16S molecule was used to identify different haplotypes and estimate their relative frequencies among 198 ticks. Eleven different haplotypes were detected. Haplotype diversity was least in northeastern collections and greatest in the southeast. The 11 haplotypes were sequenced in 24 specimens. In total, 462 bp in the 16S gene and 420 bp in the 12S gene were sequenced to reveal 66 informative sites. Phylogenetic analysis, using I. ricinus L. and I. pacificus Cooley & Kohls as outgroups, revealed 2 clades within I. scapularis. One clade was limited to the South and the other was distributed throughout the range of I. scapularis. Specimens from the Southern United States were basal in the broadly distributed clade. Random amplified polymorphic DNA by polymerase chain reaction patterns examined between members of the 2 clades provided no evidence for reproductive isolation. These patterns suggest that I. scapularis arose in the South but that a large geographic split gave rise to 2 distinct lineages. These lineages now interbreed and are partially sympatric.

Animals↗
PubMed1997

Taxonomic status of Ixodes neotomae and I. spinipalpis (Acari: Ixodidae) based on mitochondrial DNA evidence.

Ixodes spinipalpis Hadwen & Nuttall and I. neotomae Cooley are enzootic vectors of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner in western North America. The taxa overlap in host preference, habitat, and morphology. Mitochondrial DNA was compared between the taxa to test for reproductive isolation. A 300-bp region of the mitochondrial 16S ribosomal DNA gene was amplified in 28 specimens of I. neotomae and 149 specimens of I. spinipalpis. These products were screened for sequence variation using single-strand conformation polymorphism analysis, and 9 haplotypes were detected. Haplotype frequencies varied between taxa; however, Shannon diversity analysis indicated that most variation arose among collections within each taxon, and no unique haplotypes characterized either one. Phylogenetic analysis of 18 sequences, representing a replicate of each of the 9 haplotypes, was performed with Ixodes pacificus Cooley & Kohls and Ixodes jellisoni Cooley & Kohls as outgroups. Strong monophyletic support was found for a clade containing I. neotomae and I spinipalpis and within this clade no lineages comprised exclusively either taxon. These results argue against treatment of I. neotomae and I. spinipalpis as distinct species.

Animals↗
PubMed1998

Response of male Culicoides variipennis sonorensis (Diptera: Ceratopogonidae) to carbon dioxide and observations of mating behavior on and near cattle.

The response of male Culicoides variipennis sonorensis Wirth & Jones to carbon dioxide (CO2) was examined at a dairy in the Chino Basin of southern California. Males were collected using CDC-type suction traps (no light) during 6 evenings in September and October 1995 when C. v. sonorensis were abundant. Traps were baited with 1.8 kg of dry ice, 200-350 nulliparous females, or nothing. Greater than 8 times as many males were captured in CO2-baited suction traps than in either female-baited or unbaited traps. Female-baited and unbaited traps did not differ in the number of males captured. Observations of mating behavior near and on a host calf were made at a nearby dairy. Males swarmed 1-2 m downwind (east) of a restrained calf and 0.3-1.0 m above ground level. Males were also observed coupled with blood-feeding females on the calf venter (especially umbilicus and teats). Virgin female C. v. sonorensis were captured in CO2-baited suction traps and by aspiration while they were engorging on the venter of a tethered calf. There appear to be at least two mating strategies in this species: mating presumably may occur near hosts in male swarms as well as stenogamously on the venter of the host. These mating strategies may serve as prezygotic reproductive isolating mechanisms between closely related and sometimes sympatric members of the C. variipennis complex.

Animals↗
PubMed2005

Effect of seminal fluids in mating between M and S forms of Anopheles gambiae.

Previous studies have shown that sympatric populations of M and S molecular forms of Anopheles gambiae sensu stricto exhibit strong assortative mating. In the few documented cases of cross-mating between M and S forms, females that mated with amale of the alternative form were often also mated with a male of their own form. A potential explanation for the association between cross-mating and double mating could be that male accessory gland or sperm proteins that are responsible for inducing refractoriness to further mating by females have diverged between the M and S forms. This mechanism of postmating reproductive isolation would have important implications for our understanding of the speciation processes in the An. gambiae complex. We tested for this mechanism, by comparing the likelihood of mating, feeding, and laying eggs, as well as the fertility of females presented with males of their own form or the alternate form in the laboratory. We also compared the likelihood of remating in cross-mated and assortatively-mated females, and we analyzed their progeny to unravel patterns of sperm precedence. We found that cross-mated females differed from assortatively-mated females only in terms of egg-hatching rate and larval survival but that these effects could be attributed to hybrid vigor rather than differential response to seminal products. Cross-mating between forms was not associated with remating behavior. These results indicate that the sex proteins responsible for inhibiting further insemination and triggering the gonotrophic cycle in females have not diverged between these M and S populations. We discuss alternative explanations for the patterns of cross-mating and multiple mating observed in the field.

Animals↗
PubMed2004

Generic scan using AFLP markers as a means to assess the role of directional selection in the divergence of sympatric whitefish ecotypes.

Under the ecological theory of adaptive radiation, adaptation and reproductive isolation are thought to evolve as a result of divergent natural selection. Accordingly, elucidating the genetic basis of these processes is essential toward understanding the role of selection in shaping biological diversity. In this respect, the number of genes that evolved by selection remains contentious. To address this issue, the pattern of genetic differentiation obtained using 440 AFLP loci was compared with that expected under neutrality in four sympatric pairs of lake whitefish ecotypes that evolved adaptive phenotypic differences associated with the exploitation of distinct ecological niches. On average, 14 loci showed restricted gene flow relative to neutral expectation, suggesting a role of directional selection on their divergence. Among all loci that are most likely under directional selection, six exhibited parallel patterns of divergence, which provided further support for the role of selection in driving their divergence. Overall, these results indicate that only a small proportion of scored AFLP loci (between 1.4% and 3.2%) might be linked to genes implicated in the adaptive radiation of lake whitefish.

Animals↗
PubMed2005

Rapid evolution of the MH class I locus results in different allelic compositions in recently diverged populations of Atlantic salmon.

We compared major histocompatibility class I allelic diversity in two currently reproductively isolated Atlantic salmon (Salmo salar) populations (Irish and Norwegian) with a common postglacial origin in order to test for among-population differences in allelic composition and patterns of recombination and point mutation. We also examined the evidence for adaptive molecular divergence at this locus by analyzing the rate of amino acid replacement in relation to a neutral expectation. Contrary to our prediction, and in contrast to the situation for other genetic markers, the two populations have almost nonoverlapping sets of major histocompatibility class I alleles. Although there is a strong signal of point mutation that predates population divergence, recent recombination, acting in similar, but not identical, ways in both populations appears to be a significant force in creating new alleles. Moreover, selection acting on peptide-binding residues seems to favor new recombinant alleles and is likely to be responsible for the rapid divergence between populations.

Alleles↗
PubMed2005

Proceedings of the SMBE Tri-National Young Investigators' Workshop 2005. Molecular genetics of natural populations.

Significant progress in evolutionary genetics has been made by studying, on the one hand, patterns of DNA sequence polymorphism and, on the other, genetic architecture of complex adaptive traits. However, connections between nucleotide variants under selection and adaptively relevant phenotypes are missing. Such connections can be established using precise gene replacement. We review the recent successful introduction of this technique to the analysis of two evolutionarily interesting loci--Odysseus and desaturase2. Both genes have subtle phenotypes that nevertheless could be identified using gene replacement, demonstrating that effects of naturally occurring alleles can be measured in the laboratory. This is an important first step in connecting statistical signatures of selection with adaptation in nature. More candidate genes involved in adaptation, for example, through cloning of genes responsible for reproductive isolation, now need to be identified. Molecular genetic manipulation, DNA polymorphism analysis, and field studies then have to be integrated to provide fresh insights into the mechanisms of evolutionary change.

Alleles↗
PubMed2006

Adaptive evolution of fertilization proteins within a genus: variation in ZP2 and ZP3 in deer mice (Peromyscus).

Rapid evolution of reproductive proteins has been documented in a wide variety of taxa. In internally fertilized species, knowledge about the evolutionary dynamics of these proteins between closely related taxa is primarily limited to accessory gland proteins in the semen of Drosophila. Investigation of additional taxa and functional classes of proteins is necessary in order to determine if there is a general pattern of adaptive evolution of reproductive proteins between recently diverged species. We performed an evolutionary analysis of 2 egg coat proteins, ZP2 and ZP3, in 15 species of deer mice (genus Peromyscus). Both of these proteins are involved in egg-sperm binding, a critical step in maintaining species-specific fertilization. Here, we show that Zp2 and Zp3 gene trees are not consistent with trees based on nonreproductive genes, Mc1r and Lcat, where species formed monophyletic clades. In fact, for both of the reproductive genes, intraspecific amino acid variation was extensive and alleles were sometimes shared across species. We document positive selection acting on ZP2 and ZP3 and identify specific amino acid sites that are likely targets of selection using both maximum likelihood approaches and patterns of parallel amino acid change. In ZP3, positively selected sites are clustered in and around the region implicated in sperm binding in Mus, suggesting changes may impact egg-sperm binding and fertilization potential. Finally, we identify lineages with significantly elevated rates of amino acid substitution using a Bayesian mapping approach. These findings demonstrate that the pattern of adaptive reproductive protein evolution found at higher taxonomic levels can be documented between closely related mammalian species, where reproductive isolation has evolved recently.

Adaptation, Biological↗
PubMedDate not supplied

Mating propensities and variations in enzyme activities in long-term cage populations of Drosophila melanogaster.

Environment-dependent reproductive isolation was established between cage populations (Bs) of Drosophila melanogaster originated from a Greek natural population (summer 1973) and maintained for about five years under different diets (poor-rich). The detected deviation from random mating involved no homogametic or heterogametic preference but rather, a significantly increased activity of males from populations maintained on the rich food medium. This observation indicates that the male parental investment is not negligible and under certain conditions sexual isolation can be a function not only of female behavior but also of male behavior. Differences also were found in various enzyme activities on the inter- and intra-population levels. Given those observations as well as the observed different behavioral patterns of Bs and Cs-Ds populations 19, a preliminary attempt was made to associate adaptive evolution with differences in enzyme activities. The differences in enzyme activities between populations reared on different media are not due to allozymic differences. It also was shown that in some populations environmental effects do not always elicit differences in enzyme activity. It was concluded, therefore, that the observed variations were the result of environmental effects interacting with modifier genes.

Animals↗
PubMedDate not supplied

Chromosome conservation in the Bovidae.

The chromosomes of 12 bovid species were harvested from fibroblast cultures after incorporation of bromodeoxyuridine into early replicating DNA. Q-band karyotypes were constructed, and, when possible, autosomal arms were numbered according to the cattle standard karyotype. Diploid chromosome number ranged from 30 to 60, yet, based on band similarity, chromosome-arm homologies were extensive. Employing the cattle karyotype as the standard, autosomal-arm differences indicative of possible syntenic disruption were noted for only chromosomes 3, 9, and 14. While chromosome-arm homologies were extensive, shared homologous biarmed chromosomes were rare. The commonness of monobrachially homologous biarmed chromosomes among some bovids (e.g., Antilopinae) suggested that reproductive isolation and speciation in some instances might have resulted from centric fusion events.

Animals↗
PubMed2001

Polymorphism in abalone fertilization proteins is consistent with the neutral evolution of the egg's receptor for lysin (VERL) and positive darwinian selection of sperm lysin.

The evolution of species-specific fertilization in free-spawning marine invertebrates is important for reproductive isolation and may contribute to speciation. The biochemistry and evolution of proteins mediating species-specific fertilization have been extensively studied in the abalone (genus Haliotis). The nonenzymatic sperm protein lysin creates a hole in the egg vitelline envelope by species-specifically binding to its egg receptor, VERL. The divergence of lysin is promoted by positive Darwinian selection. In contrast, the evolution of VERL does not depart from neutrality. Here, we cloned a novel nonrepetitive region of VERL and performed an intraspecific polymorphism survey for red (Haliotis rufescens) and pink (Haliotis corrugata) abalones to explore the evolutionary forces affecting VERL. Six statistical tests showed that the evolution of VERL did not depart from neutrality. Interestingly, there was a subdivision in the VERL sequences in the pink abalone and a lack of heterozygous individuals between groups, suggesting that the evolution of assortative mating may be in progress. These results are consistent with a model which posits that egg VERL is neutrally evolving, perhaps due to its repetitive structure, while sperm lysin is subjected to positive Darwinian selection to maintain efficient interaction of the two proteins during sperm competition.

Amino Acid Sequence↗
PubMed2001

Genetic identification of wild and domestic cats (Felis silvestris) and their hybrids using Bayesian clustering methods.

Crossbreeding with free-ranging domestic cats is supposed to threaten the genetic integrity of wildcat populations in Europe, although the diagnostic markers to identify "pure" or "admixed" wildcats have never been clearly defined. Here we use mitochondrial (mt) DNA sequences and allelic variation at 12 microsatellite loci to genotype 128 wild and domestic cats sampled in Italy which were preclassified into three separate groups: European wildcats (Felis silvestris silvestris), Sardinian wildcats (Felis silvestris libyca), and domestic cats (Felis silvestris catus), according to their coat color patterns, collection localities, and other phenotypical traits, independently of any genetic information. For comparison, we included some captive-reared hybrids of European wild and domestic cats. Genetic variability was significantly partitioned among the three groups (mtDNA estimate of F(ST) = 0.36; microsatellite estimate of R(ST) = 0.30; P < 0.001), suggesting that morphological diversity reflects the existence of distinct gene pools. Multivariate ordination of individual genotypes and clustering of interindividual genetic distances also showed evidence of distinct cat groups, partially congruent with the morphological classification. Cluster analysis, however, did not enable hybrid cats to be identified from genetic information alone, nor were all individuals assigned to their populations. In contrast, a Bayesian admixture analysis simultaneously assigned the European wildcats, the Sardinian wildcats, and the domestic cats to different clusters, independent of any prior information, and pointed out the admixed gene composition of the hybrids, which were assigned to more than one cluster. Only one putative Sardinian wildcat was assigned to the domestic cat cluster, and one presumed European wildcat showed mixed (hybrid) ancestry in the domestic cat gene pool. Mitochondrial DNA sequences indicated that three additional presumed European wildcats might have hybrid ancestry. These four cats were sampled from the same area in the northernmost edge of the European wildcat distribution in the Italian Apennines. Admixture analyses suggest that wild and domestic cats in Italy are distinct, reproductively isolated gene pools and that introgression of domestic alleles into the wild-living population is very limited and geographically localized.

Alleles↗
PubMed2002

The plastid chromosome of Atropa belladonna and its comparison with that of Nicotiana tabacum: the role of RNA editing in generating divergence in the process of plant speciation.

The nuclear and plastid genomes of the plant cell form a coevolving unit which in interspecific combinations can lead to genetic incompatibility of compartments even between closely related taxa. This phenomenon has been observed for instance in Atropa-Nicotiana cybrids. We have sequenced the plastid chromosome of Atropa belladonna (deadly nightshade), a circular DNA molecule of 156,688 bp, and compared it with the corresponding published sequence of its relative Nicotiana tabacum (tobacco) to understand how divergence at the level of this genome can contribute to nuclear-plastid incompatibilities and to speciation. It appears that (1) regulatory elements, i.e., promoters as well as translational and replicational signal elements, are well conserved between the two species; (2) genes--including introns--are even more highly conserved, with differences residing predominantly in regions of low functional importance; and (3) RNA editotypes differ between the two species, which makes this process an intriguing candidate for causing rapid reproductive isolation of populations.

Amino Acid Sequence↗
PubMed1997

Evolution of SINE S1 retroposons in Cruciferae plant species.

The S1 element is a plant short interspersed element (SINE) that was first described and studied in Brassica napus. In this work, we investigated the distribution and the molecular phylogeny of the S1 element within the Cruciferae (= Brassicaceae). S1 elements were found to be widely distributed within the Cruciferae, especially in species of the tribe Brassiceae. The molecular phylogeny of S1 elements in eight Cruciferae species (Brassica oleracea, Brassica rapa, Brassica napus, Brassica nigra, Sinapis, arvensis, Sinapis pubescens, Coincya monensis, and Vella spinosa) was inferred using 14-36 elements per species. Significant neighbor-joining and maximum-parsimony phylogenetic clusters, supported by high bootstrap P values and/or represented in 100% of the most-parsimonious trees, were observed for each species. Most of these clusters probably correspond to recent species-specific bursts of S1 amplification. Since these species diverged recently, S1 amplification in Cruciferae plants is proposed to be a highly dynamic process that could contribute to genome rearrangements and eventually lead to reproductive isolation. S1 sequence analysis also revealed putative gene conversion events that occurred between different S1 elements of a given species. These events suggest that gene conversion is a minor but significant component of the molecular drive governing S1 concerted evolution.

Base Sequence↗
PubMed2000

Positive selection and propeptide repeats promote rapid interspecific divergence of a gastropod sperm protein.

Male-specific proteins have increasingly been reported as targets of positive selection and are of special interest because of the role they may play in the evolution of reproductive isolation. We report the rapid interspecific divergence of cDNA encoding a major acrosomal protein of unknown function (TMAP) of sperm from five species of teguline gastropods. A mitochondrial DNA clock (calibrated by congeneric species divided by the Isthmus of Panama) estimates that these five species diverged 2-10 MYA. Inferred amino acid sequences reveal a propeptide that has diverged rapidly between species. The mature protein has diverged faster still due to high nonsynonymous substitution rates (> 25 nonsynonymous substitutions per site per 10(9) years). cDNA encoding the mature protein (89-100 residues) shows evidence of positive selection (Dn/Ds > 1) for 4 of 10 pairwise species comparisons. cDNA and predicted secondary-structure comparisons suggest that TMAP is neither orthologous nor paralogous to abalone lysin, and thus marks a second, phylogenetically independent, protein subject to strong positive selection in free-spawning marine gastropods. In addition, an internal repeat in one species (Tegula aureotincta) produces a duplicated cleavage site which results in two alternatively processed mature proteins differing by nine amino acid residues. Such alternative processing may provide a mechanism for introducing novel amino acid sequence variation at the amino-termini of proteins. Highly divergent TMAP N-termini from two other tegulines (Tegula regina and Norrisia norrisii) may have originated by such a mechanism.

Acrosome↗
PubMed1995

Positive selection is a general phenomenon in the evolution of abalone sperm lysin.

Lysin is a 16kDa acrosomal protein used by abalone sperm to create a hole in the egg vitelline envelope (VE). The interaction of lysin with the VE is species-selective and is one step in the multistep fertilization process that restricts heterospecific (cross-species) fertilization. For this reason, the evolution of lysin could play a role in establishing prezygotic reproductive isolation between species. Previously, we sequenced sperm lysin cDNAs from seven California abalone species and showed that positive Darwinian selection promotes their divergence. In this paper an additional 13 lysin sequences are presented representing species from Japan, Taiwan, Australia, New Zealand, South Africa, and Europe. The total of 20 sequences represents the most extensive analysis of a fertilization protein to date. The phylogenetic analysis divides the sequences into two major clades, one composed of species from the northern Pacific (California and Japan) and the other composed of species from other parts of the world. Analysis of nucleotide substitution demonstrates that positive selection is a general process in the evolution of this fertilization protein. Analysis of nucleotide and codon usage bias shows that neither parameter can account for the robust data supporting positive selection. The selection pressure responsible for the positive selection on lysin remains unknown.

Amino Acid Sequence↗
PubMed1984

Calibrating the molecular clock: estimates of ground squirrel divergence made using fossil and geological time markers.

According to 5-Myr-old fossil evidence, ground squirrels within the genus Spermophilus had diverged into subgenera Spermophilus and Otospermophilus by late Miocene times. Radiometric dating has also provided a precise time for the sudden onset of a geological event, occurring 0.725 Myr ago, that initiated the complete and permanent reproductive isolation of two subspecies within the subgenus Otospermophilus. Since these two subspecies (S. beecheyi beecheyi and S. b. douglasii) readily hybridize with each other under laboratory conditions, allopatric subspeciation is unlikely to have occurred prior to 0.725 Myr ago. We employed Nei's model for estimating genetic distance in units which are linear in time, calibrated on the 0.725-Myr-ago date for initiation of S. b. subspeciation, to test its ability to generate a time scale for subgeneric divergence in keeping with the minimum estimate provided by the fossil record. This represents the most valid test to date of the utility of Nei's model for estimating genetic distance in units which are linear in time. Nei's model was found to underestimate this minimum time by 1 Myr, but it approximated this date after correcting values of D for variation in rates of evolution among loci.

Animals↗