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PubMed2006

The complex causality of geographical parthenogenesis.

Asexual organisms usually have larger and more northern distributions than their sexual relatives. This phenomenon, called geographical parthenogenesis, has been controversially attributed to predispositions in certain taxa; advantages of polyploidy and/or hybrid origin; better colonizing abilities because of uniparental reproduction; introgression of apomixis into sexuals; niche differentiation of clones; or biotic interactions. This review on apomictic plants demonstrates that each of these factors alone has not been able to explain the observed distributions. Establishment of the complex regulatory system of apomixis requires taxonomic and geographical predispositions; hybridization and/or polyploidization do create diversity, but they do not necessarily result in large distributions; colonizing abilities depend on clonal diversity and are outweighed by sexuals by self-compatibility and higher potentials for speciation; niche differentiation, ploidy levels and selfing keep sympatric sexuals and apomicts separated; and the impact of biotic interactions on distributions is uncertain. In conclusion, the distributional success of apomicts has a complex causality and depends on certain circumstances and combinations of factors. The rare establishment of apomixis may help to explain the predominance of sexuality on the large scale.

Genotype↗
PubMed2004

Flycatcher song in allopatry and sympatry--convergence, divergence and reinforcement.

The theory of reinforcement predicts that natural selection against the production of unfit hybrids favours traits that increase assortative mating. Whether culturally inherited traits, such as bird song, can increase assortative mating by reinforcement is largely unknown. We compared songs of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) from two hybrid zones of different ages with songs from allopatric populations. Previously, a character divergence in male plumage traits has been shown to reinforce premating isolation in sympatric flycatchers. In contrast, we find that the song of the pied flycatcher has converged towards that of the collared flycatcher (mixed singing). However, a corresponding divergence in the collared flycatcher shows that the species differences in song characters are maintained in sympatry. Genetic analyses suggest that mixed song is not caused by introgression from the collared flycatcher, but rather due to heterospecific copying. Circumstantial evidence suggests that mixed song may increase the rate of maladaptive hybridization. In the oldest hybrid zone where reinforcement on plumage traits is most pronounced, the frequency of mixed singing and hybridization is also lowest. Thus, we suggest that reinforcement has reduced the frequency of mixed singing in the pied flycatcher and caused a divergence in the song of the collared flycatcher. Whether a culturally inherited trait promotes or opposes speciation in sympatry may depend on its plasticity. The degree of plasticity may be genetically determined and accordingly under selection by reinforcement.

Animals↗
PubMed2003

Socially mediated speciation.

We employ a simple model to show that social selection can lead to prezygotic reproductive isolation. The evolution of social discrimination causes the congealing of phenotypically similar individuals into different, spatially distinct tribes. However, tribal formation is only obtained for certain types of social behavior: altruistic and selfish acts can produce tribes, whereas spiteful and mutualistic behaviors never do. Moreover, reduced hybrid fitness at tribal borders leads to the selection of mating preferences, which then spread to the core areas of the respective tribes. Unlike models of resource competition, our model generates reproductive isolation in an ecologically homogeneous environment. We elaborate on how altruistic acts can lead to reproductive isolation, but also predict that certain types of competition can lead to the speciation effect. Our theory provides a framework for how individual-level interactions mold lineage diversification, with parapatric speciation as a possible end product.

Social Behavior↗
PubMed1994

Distribution of P and hobo mobile elements in environmentally manipulated long-term Drosophila melanogaster cage populations.

The copy number and the chromosome positions of the P and hobo insertions were determined by means of in situ hybridization to polytene chromosomes, in five long-term Drosophila melanogaster cage populations kept for 18 years under different culture conditions (temperature and relative humidity). The analysis revealed that the copy number of both P and hobo elements were similar between the populations kept under the same culture conditions and significantly different among the populations maintained under different culture conditions. A tendency for similar distribution of these elements along the major chromosome arms was also observed in the populations of the same environmental manipulation. The distribution of the insertions along the chromosomes was not random for both the P and hobo elements; sites with high insertion frequencies were found (hot spots of occupation). Some of them were common in all cage populations while others were characteristic of the populations kept under the same conditions. Finally, fixed sites of occupation were also observed in all populations and refer mostly to hobo distribution. The data are discussed on the basis of the possible involvement of the P and hobo elements, in some way, to the adaptation process and speciation.

Animals↗
PubMed1990

Microorganisms associated with chromosome destruction and reproductive isolation between two insect species.

Microorganisms have been implicated in causing cytoplasmic incompatibility in a variety of insect species, including mosquitoes, fruitflies, beetles and wasps. The effect is typically unidirectional: incompatible crosses produce no progeny or sterile males, whereas the reciprocal crosses produce normal progeny. The parasitic wasp Nasonia vitripennis is one of the few species in which the cytogenetic mechanism of incompatibility is known. In this species the paternal chromosome set forms a tangled mass in a fertilized egg and is eventually lost. Here we report that cytoplasmic microorganisms are associated with complete bidirectional incompatibility between N. vitripennis and a closely related sympatric species, N. giraulti. Microorganisms can be seen in the eggs of both species. Hybrid offspring are normally not produced in crosses between the two species, but do occur after elimination of the microorganisms by antibiotic treatment. A cytogenetic and genetic study shows that bidirectional interspecific incompatibility is due to improper condensation of the paternal chromosomes. Microorganism-mediated reproductive isolation is of interest because it could provide a rapid mode of speciation. The mechanism of incompatibility in Nasonia is also of interest as a potential tool for studying chromosome imprinting and chromosome condensation.

Animals↗
PubMed2006

C. elegans CPB-3 interacts with DAZ-1 and functions in multiple steps of germline development.

Cytoplasmic polyadenylation element-binding proteins (CPEBs) are well-conserved RNA-binding proteins, which regulate mRNA translation mainly through control of poly(A) elongation. Here, we show that CPB-3, one of the four CPEB homologs in C. elegans, positively regulates multiple aspects of oocyte production. CPB-3 protein was highly expressed in early meiotic regions of the hermaphrodite gonad. Worms deficient in cpb-3 were apparently impaired in germ cell proliferation and differentiation including sperm/oocyte switching and progression of female meiosis. We also show that cpb-3 is likely to promote the meiotic entry in parallel with gld-3, a component of one of the redundant but essential genetic pathways for the entry to and progression through meiosis. Taken together, CPEB appears to have a conserved role in the early phase of meiosis and in the sperm/oocyte specification, in addition to its reported function during meiotic progression.

Animals↗
PubMed2000

Sympatric speciation: compliance with phenotype diversification from a single genotype.

A novel mechanism for sympatric speciation that takes into account complex bioprocesses within each individual organism is proposed. According to dynamical systems theory, organisms with identical genotypes can possess differentiated physiological states and may coexist 'symbiotically' through appropriate mutual interaction. With mutations, the phenotypically differentiated organisms gradually come to possess distinct genotypes while maintaining their symbiotic relationship. This symbiotic speciation is robust against sexual recombination, because offspring of mixed parentage with intermediate genotypes are less fit than their parents. This leads to sterility of the hybrid. Accordingly, a basis for mating preference also arises.

Biological Evolution↗
PubMed2005

Simultaneous Quaternary radiations of three damselfly clades across the Holarctic.

If climate change during the Quaternary shaped the macroevolutionary dynamics of a taxon, we expect to see three features in its history: elevated speciation or extinction rates should date to this time, more northerly distributed clades should show greater discontinuities in these rates, and similar signatures of those effects should be evident in the phylogenetic and phylodemographic histories of multiple clades. In accordance with the role of glacial cycles, speciation rates increased in the Holarctic Enallagma damselflies during the Quaternary, with a 4.25x greater increase in a more northerly distributed clade as compared with a more southern clade. Finer-scale phylogenetic analyses of three radiating clades within the northern clade show similar, complex recent histories over the past 250,000 years to produce 17 Nearctic and four Palearctic extant species. All three are marked by nearly synchronous deep splits that date to approximately 250,000 years ago, resulting in speciation in two. This was soon followed by significant demographic expansions in at least two of the three clades. In two, these expansions seem to have preceded the radiations that have given rise to most of the current biodiversity. Each also produced species at the periphery of the clade's range. In spite of clear genetic support for reproductive isolation among almost all species, mtDNA signals of past asymmetric hybridization between species in different clades also suggest a role for the evolution of mate choice in generating reproductive isolation as species recolonized the landscape following deglaciation. These analyses suggest that recent climate fluctuations resulted in radiations driven by similar combinations of speciation processes acting in different lineages.

Adaptation, Biological↗
PubMed1993

The genetic basis of Haldane's rule and the nature of asymmetric hybrid male sterility among Drosophila simulans, Drosophila mauritiana and Drosophila sechellia.

Haldane's rule (i.e., the preferential hybrid sterility and inviability of heterogametic sex) has been known for 70 years, but its genetic basis, which is crucial to the understanding of the process of species formation, remains unclear. In the present study, we have investigated the genetic basis of hybrid male sterility using Drosophila simulans, Drosophila mauritiana and Drosophila sechellia. An introgression of D. sechellia Y chromosome into a fairly homogenous background of D. simulans did not show any effect of the introgressed Y on male sterility. The substitution of D. simulans Y chromosome into D. sechellia, and both reciprocal Y chromosome substitutions between D. simulans and D. mauritiana were unsuccessful. Introgressions of cytoplasm between D. simulans and D. mauritiana (or D. sechellia) also did not have any effect on hybrid male sterility. These results rule out the X-Y interaction hypothesis as a general explanation of Haldane's rule in this species group and indicate an involvement of an X-autosome interaction. Models of symmetrical and asymmetrical X-autosome interaction have been developed which explain the Y chromosome substitution results and suggest that evolution of interactions between different genetic elements in the early stages of speciation is more likely to be of an asymmetrical nature. The model of asymmetrical X-autosome interaction also predicts that different sets of interacting genes may be involved in different pairs of related species and can account for the observation that hybrid male sterility in many partially isolated species is often nonreciprocal or unidirectional.

Animals↗
PubMed1985

Nucleic acid relationships among the anaerobic mycoplasmas.

The genetic relatedness between twelve selected strains among four distinct serovars of anaerobic mycoplasmas was studied using [3H]DNA-DNA hybridization, and the results were compared with data obtained from biochemical and serological tests. Radiolabelled DNA probes were prepared from five strains representing four serovars. Based on the homology results, the anaerobic mycoplasmas can be divided into five distinct groups representing five distinct species and two distinct genera. There are two species in the Anaeroplasma bactoclasticum serovar 1 group represented by strains JR and A-2, one species in serovar 2, one species in A. abactoclasticum serovar 3 and one among the unclassified serovar 4 anaerobic mycoplasmas. The probe to nonsterol-requiring strain 161 of serovar 4 showed no homology with any of the established nonsterol-requiring Acholeplasma species DNAs, or with Mycoplasma hominis DNA, or with avian DNA which served as a negative control. There was good correlation between the phenotypic and genotypic properties of the five distinct anaerobic mycoplasma species but the results indicate that phenotypic properties are not always adequate for speciation of the anaerobic mycoplasmas.

Base Composition↗
PubMed1998

Sexual conflict and speciation.

We review the significance of two forms of sexual conflict (different evolutionary interests of the two sexes) for genetic differentiation of populations and the evolution of reproductive isolation. Conflicting selection on the alleles at a single locus can occur in males and females if the sexes have different optima for a trait, and there are pleiotropic genetic correlations between the sexes for it. There will then be selection for sex limitation and hence sexual dimorphism. This sex limitation could break down in hybrids and reduce their fitness. Pleiotropic genetic correlations between the sexes could also affect the likelihood of mating in interpopulation encounters. Conflict can also occur between (sex-limited) loci that determine behaviour in males and those that determine behaviour in females. Reproductive isolation may occur by rapid coevolution of male trait and female mating preference. This would tend to generate assortative mating on secondary contact, hence promoting speciation. Sexual conflict resulting from sensory exploitation, polyspermy and the cost of mating could result in high levels of interpopulation mating. If females evolve resistance to make pre- and postmating manipulation, males from one population could be more successful with females from the other, because females would have evolved resistance to their own (but not to the allopatric) males. Between-locus sexual conflict could also occur as a result of conflict between males and females of different populations over the production of unfit hybrids. We develop models which show that females are in general selected to resist such matings and males to persist, and this could have a bearing on both the initial level of interpopulation matings and the likelihood that reinforcement will occur. In effect, selection on males usually acts to promote gene flow and to restrict premating isolation, whereas selection on females usually acts in the reverse direction. We review theoretical models relevant to resolution of this conflict. The winning role depends on a balance between the 'value of winning' and 'power' (relating to contest or armament costs): the winning role is likely to correlate with high value of winning and low costs. Sperm-ovum (or sperm-female tract) conflicts (and their plant parallels) are likely to obey the same principles. Males may typically have higher values of winning, but it is difficult to quantify 'power', and females may often be able to resist mating more cheaply than males can force it. We tentatively predict that sexual conflict will typically result in a higher rate of speciation in 'female-win' clades, that females will be responsible for premating isolation through reinforcement, and that 'female-win' populations will be less genetically diverse.

Alleles↗
PubMed2005

Interspecific divergence, intrachromosomal recombination, and phylogenetic utility of Y-chromosomal genes in Drosophila.

Reconstruction of phylogenetic relationships among recently diverged species is complicated by three general problems: segregation of polymorphisms that pre-date species divergence, gene flow during and after speciation, and intra-locus recombination. In light of these difficulties, the Y chromosome offers several important advantages over other genomic regions as a source of phylogenetic information. These advantages include the absence of recombination, rapid coalescence, and reduced opportunity for interspecific introgression due to hybrid male sterility. In this report, we test the phylogenetic utility of Y-chromosomal sequences in two groups of closely related and partially inter-fertile Drosophila species. In the D. bipectinata species complex, Y-chromosomal loci unambiguously recover the phylogeny most consistent with previous multi-locus analysis and with reproductive relationships, and show no evidence of either post-speciation gene flow or persisting ancestral polymorphisms. In the D. simulans species complex, the situation is complicated by the duplication of at least one Y-linked gene region, followed by intrachromosomal recombination between the duplicate genes that scrambles their genealogy. We suggest that Y-chromosomal sequences are a useful tool for resolving phylogenetic relationships among recently diverged species, especially in male-heterogametic organisms that conform to Haldane's rule. However, duplication of Y-linked genes may not be uncommon, and special care should be taken to distinguish between orthologous and paralogous sequences.

Animals↗
PubMed2005

Gradual genome stabilisation by progressive reduction of the Saccharomyces uvarum genome in an interspecific hybrid with Saccharomyces cerevisiae.

Considerable amounts of molecular and genetic data indicate that interspecific hybridisation may not be rare among natural strains of Saccharomyces sensu stricto. Although a post-zygotic barrier operating during meiosis usually prevents the production of viable spores, stable hybrids can arise which can even evolve into distinct species. This study was aimed to analyse the genome of a fertile Saccharomyces cerevisiae x S. uvarum hybrid and monitor its changes over four filial generations of viable spores. The molecular genetic analysis demonstrated that the two species did not contribute equally to the formation and stabilisation of the hybrid genome. S. cerevisiae provided the mitochondrial DNA and the more stable part of the nuclear genome. The S. uvarum part of the hybrid nuclear genome became progressively smaller by loosing complete chromosomes and genetic markers in the course of successive meiotic divisions. Certain S. uvarum chromosomes were eliminated and/or underwent rearrangements in interactions with S. cerevisiae chromosomes. Numerous S. uvarum chromosomes acquired S. cerevisiae telomere sequences. The gradual elimination of large parts of the S. uvarum genome was associated with a progressive increase of sporulation efficiency. We hypothesise that this sort of genomic alterations may contribute to speciation in Saccharomyces sensu stricto.

Chromosomes, Fungal↗
PubMed1979

Expermental hybridization within the genus Triturus (Urodela: Salamandridae). III. Evidence for crossing-over, true chiasmata and chomosomal homologies in the spermatogenesis of F1 species hybrids, T. cristatus carnifex female X T. marmoratus male.

Spermatogenesis in the F1 hybrid (2n=24=12 female + 12 male) between the closely related newt species T. cristatus carnifex and T. marmoratus was apparently normal up to pachytene. Many unpaired chromosomes were present at diplotene and a typical diakinesis was lacking. Primary spermatocytes at meta- and meta-anaphase contained up to 12 regular intergenomal bivalents and a corresponding number of univalents when less then 12 II. Most chiasmata were terminal or subterminal, some intercalary. Chiasmata between corresponding heterospecific chromosomes can be reported as true: real crossing over has taken place, proving the presence of primary chromosomal homologies between the 2 sets of the parental species. Evidence for recombination is based on the segregation of particular markers (i.e., subterminal C-bands and NORs) observed in certain chromosomes at metaphase II. One chromatid of single chromosomes can show the T. cristatus "pheno-type" and the other the T. marmoratus phenotype". A few primary spermatocytes contain a certain number of irregular associations (intragenomal or intrahaploid bivalents, irregular intergenomal bivalents, chromosome multivalents) joined by chiasmata which can be defined as anomalous. Other abnormalities concern the occurrence of interlocked bivalents which occasionally show an anomalous exchange between heterologous chromatids. Cytogenetic criteria useful to evaluate the taxonomic relationships between different species have been discussed as well as some possible trends in chromosome evolution and speciation within the genus Triturus.

Animals↗
PubMed2004

Differences in the rDNA-bearing chromosome divide the Asian-Pacific and Atlantic species of Crassostrea (Bivalvia, Mollusca).

Karyotype and chromosomal location of the major ribosomal RNA genes (rDNA) were studied using fluorescence in situ hybridization (FISH) in five species of CRASSOSTREA: three Asian-Pacific species (C. gigas, C. plicatula, and C. ariakensis) and two Atlantic species (C. virginica and C. rhizophorae). FISH probes were made by PCR amplification of the intergenic transcribed spacer between the 18S and 5.8S rRNA genes, and labeled with digoxigenin-11-dUTP. All five species had a haploid number of 10 chromosomes. The Atlantic species had 1-2 submetacentric chromosomes, while the three Pacific species had none. FISH with metaphase chromosomes detected a single telomeric locus for rDNA in all five species without any variation. In all three Pacific species, rDNA was located on the long arm of Chromosome 10 (10q)--the smallest chromosome. In the two Atlantic species, rDNA was located on the short arm of Chromosome 2 (2p)--the second longest chromosome. A review of other studies reveals the same distribution of NOR sites (putative rDNA loci) in three other species: on 10q in C. sikamea and C. angulata from the Pacific Ocean and on 2p in C. gasar from the western Atlantic. All data support the conclusion that differences in size and shape of the rDNA-bearing chromosome represent a major divide between Asian-Pacific and Atlantic species of CRASSOSTREA: This finding suggests that chromosomal divergence can occur under seemingly conserved karyotypes and may play a role in reproductive isolation and speciation.

Animals↗
PubMed2002

Speciation despite gene flow when developmental pathways evolve.

Evolutionary biologists assume that species formation requires a drastic reduction in gene exchange between populations, but the rate sufficient to prevent speciation is unknown. To study speciation, we use a new class of population genetic models that incorporate simple developmental genetic rules, likely present in all organisms, to construct the phenotype. When we allow replicate populations to evolve in parallel to a new, shared optimal phenotype, often their hybrids acquire poorly regulated phenotypes: Dobzhansky-Muller incompatibilities arise and postzygotic reproductive isolation evolves. Here we show that, although gene exchange does inhibit this process, it is the proportion of migrants exchanged (m) rather than the number of migrants (Nm) that is critical, and rates as high as 16 individuals exchanged per generation still permit the evolution of postzygotic isolation. Stronger directional selection counters the inhibitory effect of gene flow, increasing the speciation probability. We see similar results when populations in a standard two-locus, two-allele Dobzhansky-Muller model are subject to simultaneous directional selection and gene flow. However, in developmental pathway models with more than two loci, gene flow is more able to impede speciation. Genetic incompatibilities arise as frequent by-products of adaptive evolution of traits determined by regulatory pathways, something that does not occur when phenotypes are modeled using the standard, additive genetic framework. Development therefore not only constrains the microevolutionary process, it also facilitates the interactions among genes and gene products that make speciation more likely-even in the face of strong gene flow.

Animals↗
PubMed2005

Molecular phylogeny, recent radiation and evolution of gross morphology of the rhubarb genus Rheum (Polygonaceae) inferred from chloroplast DNA trnL-F sequences.

BACKGROUND AND AIMS: Rheum, a highly diversified genus with about 60 species, is mainly confined to the mountainous and desert regions of the Qinghai-Tibetan plateau and adjacent areas. This genus represents a good example of the extensive diversification of the temperate genera in the Qinghai-Tibetan plateau, in which the forces to drive diversification remain unknown. To date, the infrageneric classification of Rheum has been mainly based on morphological characters. However, it may have been subject to convergent evolution under habitat pressure, and the systematic position of some sections are unclear, especially Sect. Globulosa, which has globular inflorescences, and Sect. Nobilia, which has semi-translucent bracts. Recent palynological research has found substantial contradictions between exine patterns and the current classification of Rheum. Two specific objectives of this research were (1) to evaluate possible relationships of some ambiguous sections with a unique morphology, and (2) to examine possible occurrence of the radiative speciation with low genetic divergence across the total genus and the correlation between the extensive diversification time of Rheum and past geographical events, especially the recent large-scale uplifts of the Qinghai-Tibetan Plateau. METHODS: The chloroplast DNA trnL-F region of 29 individuals representing 26 species of Rheum, belonging to seven out of eight sections, was sequenced and compared. The phylogenetic relationships were further constructed based on the sequences obtained. KEY RESULTS: Despite the highly diversified morphology, the genetic variation in this DNA fragment is relatively low. The molecular phylogeny is highly inconsistent with gross morphology, pollen exine patterns and traditional classifications, except for identifying all samples of Sect. Palmata, three species of Sect. Spiciformia and a few species of Sect. Rheum as corresponding monophyletic groups. The monotypic Sect. Globulosa showed a tentative position within the clade comprising five species of Sect. Rheum. All of the analyses revealed the paraphyly of R. nobile and R. alexandrae, the only two species of Sect. Nobilia circumscribed by the possession of large bracts. The crude calibration of lineages based on trnL-F sequence differentiation implied an extensive diversification of Rheum within approx. 7 million years. CONCLUSIONS: Based on these results, it is suggested that the rich geological and ecological diversity caused by the recent large-scale uplifts of the Qinghai-Tibetan Plateau since the late Tertiary, coupled with the oscillating climate of the Quaternary stage, might have promoted rapid speciation in small and isolated populations, as well as allowing the fixation of unique or rare morphological characters in Rheum. Such a rapid radiation, combined with introgressive hybridization and reticulate evolution, may have caused the transfer of cpDNA haplotypes between morphologically dissimilar species, and might account for the inconsistency between morphological classification and molecular phylogeny reported here.

Biological Evolution↗