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PubMed1989

Cross-fertilization between Syrian and Chinese hamsters.

The role of the zona pellucida in the specificity of fertilization was studied by cross-inseminations between Syrian (Golden) and Chinese hamster gametes. Cumulus-enclosed eggs from both Syrian and Chinese hamsters were placed together in one dish and inseminated with spermatozoa from either one or the other species. Fertilization always took place between gametes of homologous species. Chinese hamster spermatozoa failed to bind to the zona pellucida of Syrian hamster eggs; hence, fertilization was never observed. However, Chinese hamster spermatozoa could fertilize zona-free Syrian hamster eggs. In the reciprocal cross, a large number of Syrian hamster spermatozoa could bind to and penetrate the zonae of Chinese hamster eggs. However, fusion of Syrian hamster spermatozoa with the vitellus of zona-intact Chinese hamster eggs was never observed. After removal of the zona pellucida, only a small percentage (31%) of Syrian hamster spermatozoa could fuse with Chinese hamster vitelli. Thus, in these species, the mechanisms of interspecific gamete recognition and the prevention of interspecies fertilization seem to differ according to the direction of the cross. In Syrian hamster eggs, the block to interspecies fertilization seems to exist at the level of the zona pellucida, while in Chinese hamster eggs the block is at the level of the egg plasma membrane. The implications of these results in analyses of the genetics of spermatozoa, the molecular basis of sperm-egg recognition, and mechanisms of reproductive isolation leading to speciation, are discussed.

Animals↗
PubMed2006

Heritability and heterochrony of polychromatism in a Lake Victoria Cichlid fish: Stepping stones for speciation?

In many haplochromine cichlid fish, male nuptial coloration is subject to female mate choice and plays a central role in the evolution of reproductive isolation between incipient species. Intraspecific variation in male coloration may serve as a target for diversifying sexual selection and provide a starting point for species divergence. Here, we investigated a polychromatism in Neochromis omnicaeruleus, a haplochromine from Lake Victoria, East-Africa. In this species, male coloration ranges from skyblue to yellow-red and females are grey-blue to yellow. We found that both genetic and environmental factors influence the expression of these colours during individual development. In a natural population, we found that male colour was associated with size and sexual maturity: yellow males were smaller than blue males and tended to be sexually immature. In females, size and maturity did not differ between colour types. Laboratory crosses revealed that there is a heritable component to the observed colour variation: yellow parents produced more yellow offspring than blue parents. Together with repeated aquarium observations of yellow individuals that gradually become blue, these data suggest that yellow males change to blue as they approach sexual maturity, and that the occurrence and timing of this transition is influenced by both environmental and genetic effects. The significance of this mechanism of colour expression as a possible target for divergent selection remains to be evaluated.

Africa, Eastern↗
PubMed2006

Gamete interaction: is it species-specific?

Reproductive isolation is pivotal to maintain species separation and it can be achieved through a plethora of mechanisms. In addition, the development of barriers to gamete interaction may drive speciation. Such barriers to interspecific gamete interaction can be prezygotic or postzygotic. Considering the great diversity in animal species, it is easy to assume that regulation of the early steps of fertilization is critical to maintain species identity. One prezygotic mechanism that is often mentioned in the literature is that gamete interaction is limited to gametes of the same species. But do gametes of all animals interact in a species-specific way? Are gamete interactions completely species-specific or perhaps just species-restricted? In species in which species-restrictions have been described, is the interspecies barrier at one major step in the fertilization process or is it a combination of partially restricted steps that together lead to a block in interspecific fertilization? Are the mechanisms used to avoid interspecific crosses different between free-spawning organisms and those with internal fertilization? This review will address these questions, focusing on prezygotic barriers, and will describe what is known about the molecular biology that may account for species-limited gamete recognition and fertilization.

Animals↗
PubMed1998

Population differences in a lizard communicative display: evidence for rapid change in structure and function.

Population differences in a communicative display can lead to reproductive isolation and speciation, and are an indicator of the potential for rapid change in the display. Herein, we describe differences in the push-up displays produced by three populations of sagebrush lizards, Sceloporus graciosus, in the field. Lizards from these three populations differ in the use of display body postures, the number of legs extended to produce the up-and-down motion of the display, and the number of headbobs produced in each of three segments of the display. In addition, there is at least one behaviour that is unique to one of the three populations (the 'nodding run'). These differences among populations suggest that both the structure and the function of the push-up display are undergoing far more rapid change than was previously thought. Copyright 1998 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1999

Mate choice in divergent morphs of the gastropod mollusc Littorina saxatilis (Olivi): speciation in action?

We investigated mate choice in the gastropod Littorina saxatilis (Olivi) in the laboratory, using snails taken from two shores, 20 km apart. The snails occur in two distinct shell morphs in the high (H) and mid (M) shore zones, known to show signs of a postzygotic reproductive barrier between them. We found that mating was assortative not only between the forms H and M on a single shore, but also between snails from the two different shores. Mating preference appeared to be based on morph type over this distance. The snails may be detecting and responding to an extrinsic attribute reflecting a common microhabitat (but across a distance of 20 km), and/or an intrinsic attribute to do with the organisms themselves. These findings are further evidence of behaviour associated with the evolution of reproductive isolation in this gastropod species. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1998

Molecular definition of pericentric inversion breakpoints occurring during the evolution of humans and chimpanzees.

High-resolution G-banding analysis has demonstrated remarkable morphological conservation of the chromosomes of the Hominidae family members (humans, chimpanzees, gorillas, and orangutans), with the most notable differences between the genomes appearing as changes in heterochromatin distribution and pericentric inversions. Pericentric inversions may have been important for the establishment of reproductive isolation and speciation of the hominoids as they diverged from a common ancestor. Here the previously published primate karyotype comparisons, coupled with the resources of the Human Genome Project, have been used to identify pericentric inversion breakpoints seen when comparing the human karyotype to that of chimpanzee. Yeast artificial chromosome (YAC) clones were used to detect, by fluorescence in situ hybridization, five evolutionary pericentric inversion breakpoints present on the chimpanzee chromosome equivalents of human chromosomes 4, 9, and 12. In addition, two YACs from human 12p that detect a breakpoint in chimpanzee detect a similar rearrangement in gorilla.

Animals↗
PubMed1993

Gametic imprinting as a speciation mechanism in mammals.

Gametic imprinting renders mammalian male and female genomes functionally hemizygous at specific loci. Successful embryogenesis can only be accomplished in the presence of both parental genomes. The mechanism of gametic imprinting is at present unknown, although circumstantial evidence suggests it is similar in nature to X inactivation. A model is presented which describes the effects of switching the imprint pattern at male and female target genes. The consequence of such an event is reproductive isolation, and could lead to the formation of a new species.

Animals↗
PubMed1995

Conditions for soft selection favoring the evolution of hybrid inviability.

Hybrid inviability per se is not generally considered adaptive; it is believed to evolve as a pleiotropic consequence of other genetic changes, either adaptive or neutral, between diverging populations. However, given the pre-existence of hybrid sterility, there are circumstances under which hybrid inviability may be directly selected. In this report, we model the evolution of hybrid inviability in the context of within-family density-dependent selection (soft selection). This paper shows that under certain conditions, an allele causing hybrid inviability can increase when rare, despite also decreasing viability in conspecifics. These conditions depend upon the strength of density effects within families and upon the frequency of matings with heterospecifics. It is found that, under the most favorable conditions (strong soft selection and frequent hybridization), the inviability allele can invade when its deleterious effect on the hybrid viability exceeds four times its deleterious effects on conspecific heterozygotes. The conditions for fixation of the allele are also investigated and the circumstances under which this process might operate in natural populations are discussed. The conclusion is that the most restrictive condition is that for this process to evolve, there must be little opportunity for postzygotic reproductive isolation to evolve.

Alleles↗
PubMed1995

A genetical theory of species selection.

Species selection, differential rates of speciation or extinction resulting from species level characters, is often invoked as the main mechanism of macroevolution that is not simply an extension of microevolutionary processes. So long as we are careful in defining "species", the logic of species selection is sound. This does not mean, however, that this process can influence evolutionary dynamics under realistic conditions. The principal challenge to the efficacy of species selection as an evolutionary mechanism is the idea that selection between individuals within species will be so much more efficient as to swamp out any effects of selection between species. To assess this, a genetic model is constructed that includes simultaneous selection within and between species, and this is used to ask: under what conditions could species selection influence evolutionary dynamics, even in the face of opposing selection between individuals? The model shows that the efficacy of species selection is strongly determined by the time between speciation events (measured in individual generations), the mutation rate of the character under consideration, and the initial size of a newly formed reproductively isolated population. Data indicate that a few studied lineages have shown sufficiently high speciation rates to make species selection an important mechanism in the evolution of characters with mutation rates on the order of 10(-6) per generation. Quantitative characters, such as body size, generally change too readily for species selection to be relevant to their evolution. Complex characters, however, may be good candidates to be influenced by species selection. The interaction of selection within and between species can be subtle, with individual selection looking, from the standpoint of a species, very much like development of an individual. Furthermore, selection between individuals may be the main process assembling complex adaptations, while species selection allows them to persist over long periods of time.

Animals↗
PubMed1998

Dynamics of cytonuclear disequilibria in subdivided populations.

The purpose of this report is to understand the effects of hybrid zone architecture and genetic drift on cytonuclear genotypic disequilibria. We also study the exact dynamics of the expected values of the cytonuclear genotypic disequilibria for both the homozygotes and heterozygotes in a finite population containing reproductively isolated subpopulations under random drift alone and random drift along with mutation, respectively. We study the dynamics of the between-population component of the disequilibrium for the homozygotes. The dynamics of the variances of these disequilibria under the random drift model are studied by Monte Carlo simulations. The asymptotic formulas for both the expectations and variances are obtained. The results are contrasted with those for a single undivided population and their biological significances are discussed. Approximate normality of the disequilibria measures are also demonstrated by Monte Carlo simulations.

Animals↗
PubMed2000

Rapid speciation via parallel, directional selection on regulatory genetic pathways.

Regulatory genetic pathways are ubiquitous in organisms and play a central role in the realization of the phenotype during development. We explored the proposition that these pathways can provide a plausible source of the epistatic variation that has been implicated in the evolution of postzygotic reproductive isolation. We modeled gene regulation as a matching function between the product of one locus and the promoter site of the next locus in the pathway, with binding strength determining the amount of product. When the phenotype is subject to parallel selection in a pair of independent populations, we find that the fitnesses of F(1)and F(2)hybrids often drop to very low values as the populations respond in genetically different and incompatible ways. The simulations support the predictions of the analytical models. Hybrid fitness reduction occurs more often as the number of loci in the pathway increases, and as the binding site interactions become more complex. Less hybrid fitness reduction is seen when the populations start with imperfect binding in the pathway. In contrast, when we constructed the phenotype without gene regulation using multiplicative rules, isomorphic to the additive phenotype commonly assumed in evolutionary models, we found no appreciable F(1)fitness reduction and only slight F(2)fitness reduction. The interaction of genetic drift and mutation, even at very high rates, did not reduce hybrid fitness at all on the time-scales we considered. Clearly, the evolution of regulatory genetic pathways can play an important role in speciation, but much more empirical information is needed on the effect of allelic variability in regulatory site interactions before this role is fully understood.

Animals↗
PubMed2001

Differences in (G+C) content between species: a commentary on Forsdyke's "chromosomal viewpoint" of speciation.

Forsdyke (1999) has recently argued that differences in (G+C)%, or G+C content, may trigger new species formation. He further argues that the genic model has shortcomings that can be overcome by his "chromosomal" (hereafter, "G+C") model. We disagree on several counts. First, we do not accept that the genic model has the shortcomings suggested by Forsdyke. There is an abundance of empirical support for the contribution of individual genes, as well as of mapped chromosomal regions, to post-zygotic reproductive isolation (and Haldane's rule). Further, we argue that the G+C model suffers from the same theoretical difficulties as other speciation models based on underdominance. We also question the evidence Forsdyke uses to support his model. Finally, we describe analyses of G+C content in a well-studied model system of speciation (the Drosophila melanogaster species complex), the results of which are incompatible with the G+C model. Thus, while Forsdyke's G+C model cannot be explicitly ruled out, it is not directly supported by empirical data. In contrast, the genic model is well supported by empirical data, holds up on theoretical grounds, and does not require any assistance from the G+C model.

Animals↗
PubMed1996

Genetic divergence and speciation in an extremely young species flock in Mexico formed by the genus Cyprinodon (Cyprinodontidae, Teleostei).

The lake Laguna Chichancanab contains one of the evolutionarily youngest species flocks known, composed of five endemic species of the genus Cyprinodon. The presumed sister species Cyprinodon artifrons still exists in marine coastal habitats. Sequences of the mitochondrial control region were obtained for C. artifrons (genes of 17 individuals sequenced) and for all five endemic species (genes of 32 individuals sequenced). While nine different haplotypes were found in C. artifrons, only a single haplotype was shared with the species from the laguna. Four additional haplotypes were found among the endemics. These four were most closely related to that single haplotype shared with C. artifrons. The deprivation in haplotypes among the endemics points to a small founder population or to bottlenecking. The distribution pattern of overlapping haplotypes in four of the five species suggests incomplete lineage sorting or hybridization among them. Complete reproductive isolation is only assumed for one species, C. maya, in which a single and unique haplotype was found.

Animals↗
PubMed1998

Cryptic species in a "living fossil" lineage: taxonomic and phylogenetic relationships within the genus Lepidurus (Crustacea: Notostraca) in North America.

Lineages which exhibit little morphological change over geologic time are evolutionarily and ecologically interesting, but often taxonomically difficult. For some morphologically conservative groups, not only is it almost impossible to identify significant changes in fossil forms over time, but the relationships among extant populations are often poorly understood due to lack of known characters which clearly delimit species. Notostracan crustaceans are a classic example of such "living fossils." The paradoxical characteristics of long-term stasis in gross morphology and hypervariability of many individual morphological characters make notostracans an especially taxonomically challenging group. We used molecular and biochemical techniques to investigate the taxonomic and phylogenetic relationships of four nominal species within the genus Lepidurus in North America, three of which had been alternately abandoned, resurrected, or synonymized under a single, globally distributed morphospecies since their original descriptions in the 1800s. Data from a 330-bp sequence of the mitochondrial 12S rDNA gene and from nine allozyme loci consistently indicate five highly genetically divergent clades among the populations we used to represent the four nominal species. Diagnostic molecular characters, magnitudes of genetic divergence among clades, and the fact that these genetically distinct clades have broadly overlapping geographical ranges strongly suggest that the five clades are reproductively isolated species. One of the nominal species (L. couesii) is not monophyletic, but rather consists of two species which are not sister taxa. The other three nominal species (L. lemmoni, L. packardi, and L. bilobatus) are supported as valid phylogenetic species. The best current hypothesis for phylogenetic relationships among the five species is provided by a simultaneous analysis of both 12S rDNA and allozyme data, which places L. bilobatus and L. "couesii"-1 as sister taxa and L. "couesii"-2 as the most basal of all the Lepidurus species included in this study. These results point to the existence of cryptic species within the current classification scheme for Lepidurus, the need for further taxonomic work within the Notostraca in general, and the role that genetic techniques can play in clarifying the systematics of morphologically conservative groups.

Animals↗
PubMed2002

Aspects of the ecology of Fusarium toxins in cereals.

Species of the genus Fusarium account for three of the five agriculturally important mycotoxins which are deoxynivalenol, aflatoxin, fumonisin, zearalenone and ochratoxin. The toxigenic fusaria have been complicated to study because morphologically-similar strains represent different biologies: saprophytes, pathyotypes and endophytes. This might explain the difficulties with systems of taxonomy for Fusarium species and increasing reliance on molecular techniques to characterize taxa. Another remarkable feature of the toxigenic fusaria is that each species produces compounds that cross several species as well as families of compounds that are species specific. In addition, reproductively-isolated strains (from different continents) of important species such as F. graminearum produce different compounds, and even produce the same compounds by different biosynthetic pathways.

Edible Grain↗
PubMed1996

Wolbachia: intracellular manipulators of mite reproduction.

Cytoplasmically transmitted Wolbachia (alpha-Proteobacteria) are a group of closely related intracellular microorganisms that alter reproduction in arthropods. They are found in a few isopods and are widespread in insects. Wolbachia are implicated as the cause of parthenogenesis in parasitic wasps, feminization in isopods and reproductive (cytoplasmic) incompatibility in many insects. Here we report on the widespread occurrence of Wolbachia in spider mites and predatory mites based on a PCR assay for a 730 bp fragment of the ftsZ gene with primers that are specific for Wolbachia. An additional PCR, using two primer pairs that amplify a 259 bp region of the ftsZ gene that are diagnostic for the two Wolbachia subdivisions A and B, showed that infected mites only carried type B and not type A Wolbachia. The fact that some species tested negative for Wolbachia does not mean that the entire species is uninfected. We found that natural populations of Tetranychus urticae are polymorphic for the infection. The possible effects of Wolbachia on mite reproduction and post-zygotic reproductive isolation are discussed.

Animals↗
PubMed1987

Isozymes as bioprobes for genetic analysis of nonhuman primates.

The identification and the utilization of genetically determined electrophoretic differences of enzymes between the individuals of species as well as between cell lines have played an important role in the advancement of mammalian genetics during the past quarter of a century. In an explicit search we found a number of red cell enzyme polymorphisms in each of the following four species: chimpanzees, orang utans, rhesus monkeys and brown capuchins. Allelic distribution patterns among populations have indicated trends of subspeciation among chimpanzees and orang utans due to geographic barriers leading to reproductive isolation. Investigations of quantitative levels of red cell glucose-6-phosphate dehydrogenase have suggested that relative activity profiles of certain enzymes among species may be helpful in studies of the evolution of physiological traits and their biological significance during speciation. A large number of biochemical genetic markers in primate-rodent (i.e., chimpanzee-, gorilla-, orang utan-, rhesus monkey- and African green monkey-Chinese hamster) somatic cell hybrids have been identified and are useful for primate genetic analysis. Some of the biologically relevant observations on the enzyme markers in the above mentioned primate species are discussed.

Animals↗
PubMed1993

Molecular diversity at the self-incompatibility locus is a salient feature in natural populations of wild tomato (Lycopersicon peruvianum).

A cDNA encoding a stylar protein was cloned from flowers of self-incompatible wild tomato (Lycopersicon peruvianum). The corresponding gene was mapped to the S locus, which is responsible for self-incompatibility. The nucleotide sequence was determined for this allele, and compared to other S-related sequences in the Solanaceae. The S allele was used to probe DNA from 92 plants comprising 10 natural populations of Lycopersicon peruvianum. Hybridization was conducted under moderate and permissive stringencies in order to detect homologous sequences. Few alleles were detected, even under permissive conditions, underscoring the great sequence diversity at this locus. Those alleles that were detected are highly homologous. Sequences could not be detected in self-incompatible Nicotiana alata, self-compatible L. esculentum (cultivated tomato) or self-compatible L. hirsutum. However, hybridization to an individual of self-incompatible L. hirsutum revealed a closely related sequence that maps to the S locus in this reproductively isolated species. This supports the finding that S locus polymorphism predates speciation. The extraordinarily high degree of sequence diversity present in the gametophytic self-incompatibility system is discussed in the context of other highly divergent systems representing several kingdoms.

Alleles↗