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Differential strength of sex-biased hybrid inferiority in impeding gene flow may be a cause of Haldane's rule.
In animals, if one sex of the F1 hybrid between two species is sterile or inviable, it is usually the heterogametic (XY or WZ) sex. This phenomenon, known as Haldane's rule, is currently thought to be coincidentally caused by different mechanisms in separate entities. The following questions have never been asked: Are heterogametic and homogametic inferiority (sterility or inviability) equivalent as isolating mechanisms? Could discrepancies between them, if existing, produce Haldane's rule? Here I consider sex-biased hybrid inferiority strictly as an isolating mechanism, and quantitatively evaluate its strength in impeding gene flow. The comparison reveals that the ability of sex-biased inferiority to impede gene flow varies according to the sex and chromosome involved. Heterogametic inferiority is a weaker barrier when unidirectional and a much stronger one when in compound reciprocal directions, compared with homogametic inferiority. Such differential strength may affect divergence in speciation and produce Haldane's rule.
Relative size and evolution of the germline repertoire of T-cell receptor beta-chain gene segments in nonhuman primates.
The mammalian T-cell receptor (TCR) gene complexes exist as multiple tandemly arrayed gene segments that have apparently arisen by gene duplication mechanisms. A study of the number of TCR germline gene segments in several primate species might provide insight into the relative rate and patterns of gene duplication and deletion within these gene complexes. DNA probes from the TCR beta-chain variable (TCRBV) region gene segment subfamilies 1 through 25 and the constant region gene segment were sequentially hybridized under low stringency to Southern blots containing genomic DNA of human, gorilla, orangutan, and pig-tailed macaque. The number of gene members in each subfamily was estimated from the number of hybridizing DNA fragments. The results show apparent examples of both TCRB V gene duplication and deletion since speciation of the Hominoids from Cercopithecoid (Old World) primates. For one putative duplication/deletion event involving six TCRBV gene segments, derivation and comparison of germline DNA sequence from macaque and human as well as Southern blot analysis of additional primates demonstrated that this event was a duplication that occurred after the divergence of the family Pongidae (Greater Apes) from Hylobatidae (Lesser Apes). Southern blot analysis of multiple pig-tailed macaques and their offspring suggests a degree of DNA sequence variability in these gene segments similar to that observed in humans. An appreciation of the size and variability of each TCRBV subfamily will be useful when considering the DNA primers and probes necessary to measure the relative usage of these TCRBV genes as part of the immune response in these nonhuman primates.
Critical review of host specificity and its coevolutionary implications in the fig/fig-wasp mutualism.
Figs (Ficus spp., Moraceae) and their pollinating wasps (Agaonidae, Chalcidoidea) constitute perhaps the most tightly integrated pollination mutualism that is known. Figs are characterized by extraordinarily high global and local species diversity. It has been proposed that the diversification of this mutualism has occurred through strict-sense coadaptation and cospeciation between pairs of fig and wasp species that are associated in highly specific one-to-one relationships. However, existing studies cast doubt on the generality of this proposition. Here, we review our current knowledge of the evolutionary history of the fig/fig-wasp mutualism. We critically examine the idea that codivergence between figs and their pollinators has been dominated by strict-sense cospeciation. We present phylogenetic and population genetic data from neotropical fig and fig wasp species that suggest that a more accurate model for diversification in this mutualism is that of groups of genetically well defined wasp species coevolving with genetically less well defined (frequently hybridizing) groups of figs. Last, we use our results to assess previously proposed hypotheses on models of speciation in this mutualism.
Characterization of Wolbachia infections and interspecific crosses of Aedes (Stegomyia) polynesiensis and Ae. (Stegomyia) riversi (Diptera: Culicidae).
Prior studies have identified a complicated pattern of interspecific hybridization between members of the Aedes (Stegomyia) scutellaris (Walker) mosquito group, which includes medically important vectors of bancroftian filariasis and dengue. Here, we report that two members of the group, Aedes polynesiensis Marks and Aedes riversi Bohart & Ingram, are both infected with intracellular Wolbachia bacteria. Sequencing of the Wolbachia wsp gene demonstrates that the infections differ from each other and from Wolbachia infections previously reported in mosquitoes. Aedes polynesiensis is the first mosquito identified with a wMel Wolbachia type. Intraspecific crosses of infected and aposymbiotic lines generated via antibiotic treatment show that the Wolbachia infections in both species cause high levels of cytoplasmic incompatibility. Interspecific crosses show that the two species are reproductively isolated. However, repeating the interspecific crosses with aposymbiotic mosquito strains demonstrates that the Wolbachia infections play a role in preventing hybrid offspring. We discuss Wolbachia infections in relation to better defining the evolutionary relationships and causes of speciation within the group, understanding the basis for the observed east-to-west gradient in filarial refractoriness, and developing novel genetic control measures.
Apomixis and cassava.
Apomixis means seed formation without fertilization. In cassava (Manihot esculenta) it is an alternative to reproduction by cuttings, which normally transmits pathogens and leads to an accumulation of viral and bacterial diseases. Apomixis also assures preservation of heterosis and avoids genetic segregation. It occurs in wild relatives of cassava and has been transferred successfully from Manihot glaziovii and M. neusana. It is facultative, and occurs at a low frequency, ranging from 1-2%, and apparently is genetically different from apomixis in other crops. With selection, the frequency can reach 13%. Apomixis in cassava is frequently associated with aneuploidy but it does occur in some diploid types. It is due to the formation of aposporic sacs, which can easily be detected by clearing tissue preparations. Apomixis appears to have played an important role in speciation during the evolution of Manihot, since it leads to the maintenance and perpetuation of sterile interspecific hybridization. The use of apomixis in cassava breeding could lead to a boom in line improvement and commercial production. In addition to preserving superior genotypes, avoiding contamination of new plants, it would enable international programs to export their germplasm to destination countries. This would allow the use of superior genotypes even if apomixis occurs at a low frequency. A scheme to maximize benefits is to use diploid apomictic clones as maternal parents, which can be crossed with pollinators of polyploid interspecific hybrids, followed by selection among the progeny of new apomictic types that combine the heteroses of both interspecific hybridization and polyploidy. In addition, they acquire favored genes that have been transferred from the wild to the commercial crop.
[Genetic defects in inheritance and the variability of locus Ldh-B in hybrid populations of Rana esculenta complex (Amphibia, Ranidae)].
The directed ingression of Lhd-B gene from R. lessonae to R. ridibunda observed during hybridization between these species was confirmed in the present study. The ingression was accompanied by decreased of the initial allele activity, segregative disturbances in heterozygotes and changes in electrophoretic mobility. The population genetic analysis has shown stable preservation of abnormal alleles in hybrid populations. The results are discussed with reference to the concept of genetic instability, its application to speciation models, and the neutralist vs. adaptationist controversy. Our data supports the neutral concept of allozyme variation.
Evaluation of the courtship and of the hybrid male sterility among Drosophila buzzatii cluster species (Diptera, Drosophilidae).
In the Drosophila repleta group the establishment of subgroups and complexes made on the basis of morphological and cytological evidences is supported by tests of reproductive isolation. Among species in the repleta group, the buzzatii cluster, due to its polymorphism and polytipism, is an excellent material for ecological and speciation studies. Some interspecific crosses involving Drosophila seriema, Drosophila sp. B, D. koepferae and D. buzzatii strains were completely sterile while others involving strains from these species produced F1 hybrids that did not yield F2. In the present work, data on courtship duration and copula occurrence obtained in the analysis of flies from parental sterile crosses and on spermatozoon mobility observed in F1 hybrids that did not yield F2 are presented. Copula did not occur during one hour of observation and the spermatozoon also did not show mobility at any of the analyzed stages (3, 7, 9 and 10 days old). There was a high variation in courtship average duration and in the percentage of males that courted the females. The reproductive isolation mechanisms indicated by these observations were pre and post-zygotic, as supported by the absence of copula and male sterility. Data obtained also showed the occurrence of different degrees of reproductive compatibility among the strains classified as the same species but from distinct geographic localities.
Intersectional gene flow between insular endemics of Ilex (Aquifoliaceae) on the Bonin Islands and the Ryukyu Islands.
Hybridization and introgression play important roles in plant evolution, and their occurrence on the oceanic islands provides good examples of plant speciation and diversification. Restriction fragment length polymorphisms (RFLPs) and trnL (UAA) 3'exon-trnF (GAA) intergenic spacer (IGS) sequences of chloroplast DNA (cpDNA), and the sequences of internal transcribed spacer (ITS) of nuclear ribosomal DNA were examined to investigate the occurrence of gene transfer in Ilex species on the Bonin Islands and the Ryukyu Islands in Japan. A gene phylogeny for the plastid genome is in agreement with the morphologically based taxonomy, whereas the nuclear genome phylogeny clusters putatively unrelated endemics both on the Bonin and the Ryukyu Islands. Intersectional hybridization and nuclear gene flow were independently observed in insular endemics of Ilex on both sets of islands without evidence of plastid introgression. Gene flow observed in these island systems can be explained by ecological features of insular endemics, i.e., limits of distribution range or sympatric distribution in a small land area.
Genetic basis and evolution of species-specific courtship song in the Drosophila auraria complex.
The interpulse interval (IPI) of courtship song in the Drosophila auraria complex is the only parameter that is consistently species-specific among the several courtship elements examined within the complex. The genetic basis of the species-specific courtship song was examined by analysing the song of interspecific hybrids and of backcross progeny. IPI of all interspecific hybrids except two showed intermediate values, suggesting autosomal control of species-specific IPI. However, significant deviation for shorter IPI from midparent was found in thirteen out of 20 crosses. The chromosomal analysis between D. auraria and D. biauraria revealed that the two major autosomes had significantly large effects on IPI, but the sex chromosome and cytoplasm had no effect. Since no interaction was detected, it is concluded that each autosome acts additively in the determination of species-specific IPI. The common ancestors of the D. auraria complex may also have had autosomal control of IPI, which has been conserved during speciation in the complex.
The evolution of postzygotic isolation: accumulating Dobzhansky-Muller incompatibilities.
Hybrid sterility and inviability often result from the accumulation of substitutions that, while functional on their normal genetic backgrounds, cause a loss of fitness when brought together in hybrids. Previous theory has shown that such Dobzhansky-Muller incompatibilities should accumulate at least as fast as the square of the number of substitutions separating two species, the so-called snowball effect. Here we explicitly describe the stochastic accumulation of these incompatibilities as a function of time. The accumulation of these incompatibilities involves three levels of stochasticity: (1) the number of substitutions separating two allopatric lineages at a given time; (2) the number of incompatibilities resulting from these substitutions; and (3) the fitness effects of individual incompatibilities. Previous analyses ignored the stochasticity of molecular evolution (level 1) as well as that due to the variable effects of incompatibilities (level 3). Here we approximate the full stochastic process characterizing the accumulation of hybrid incompatibilities between pairs of loci. We derive the distribution of the number of incompatibilities as a function of divergence time between allopatric taxa as well as the distribution of waiting times to speciation by postzygotic isolation. We provide simple approximations for the mean and variance of these waiting times. These results let us estimate. albeit crudely, the probability, p, that two diverged sites from different species will contribute to hybrid sterility or inviability. Our analyses of data from Drosophila and Bombina suggest that p is generally very small, on the order of 10(-6) or less.
Speciation of organisms within the Mycobacterium avium-Mycobacterium intracellulare-Mycobacterium scrofulaceum (MAIS) complex based on restriction fragment length polymorphisms.
A DNA probe which hybridizes to all pathogenic species of slow-growing mycobacteria has been used to identify restriction-fragment-length-polymorphisms (RFLPs) in Bam Hl digests of chromosomal DNA from members of the Mycobacterium avium-Mycobacterium intracellulare-Mycobacterium scrofulaceum complex. The RFLP patterns so produced were found to fall into distinct categories which were representative of each of the three species. Except for two doubtful isolates, strains of M. avium were found to fall into two related RFLP-types, one of which contained the vast majority of the strains tested. In contrast, M. intracellulare strains were found to be more heterogeneous. For these strains, we found one major RFLP-type and one subsidiary type which appears to be a sub-set of the first. We also found two further RFLP-types which contained serovars 7 and 18 respectively. We conclude from this that M. avium, M. intracellulare and M. scrofulaceum are three distinct species and that serovars belonging to the 'intermediate group' of Meissner and Anz belong to the species M. avium. Utilizing these criteria, we examined a number of isolates from the 'ambiguous' serovar 9 and found that of eight strains tested, six typed as M. avium and two typed as M. intracellulare.
Reproductive isolation in Drosophila: how close are we to untangling the genetics of speciation?
Our understanding of the genetic basis of reproductive isolation in Drosophila has progressed rapidly over the past decade. Details of the genetic structure of hybrid sterility have been revealed and a general consensus has been reached concerning the genetic bases of Haldane's rule. Genetic analyses now reach beyond hybrid sterility and inviability, allowing us to make important comparisons across different traits involved in reproductive isolation. Expansion of genetic studies to include rescue of hybrid incompatibilities has opened the door for more detailed molecular and developmental analyses of reproductive isolation than has ever before been possible.
Forty years of solitude: life-history divergence and behavioural isolation between laboratory lines of Drosophila melanogaster.
The study of the early stages of speciation can benefit from examination of differences between populations of known history that have been separated for a short time, such as a few thousands of generations. We asked whether two lines of Drosophila melanogaster that were isolated more than 40 years ago have evolved differences in life-history characters, or have begun to evolve behavioural or postzygotic isolation. One line, which is resistant to DDT, showed lower egg production and a shorter lifespan than a susceptible line. These differences are not a pleiotropic effect of resistance because they are not attributable to the chromosome that contains the resistance factors. The two lines have begun to become behaviourally isolated. Again, the isolation is not attributable to genes on the chromosome that contains resistance factors. The lines show only prezygotic isolation; there is no evidence of reduced fitness of F1 or F2 hybrids. These lines and others like them, should be excellent subjects for analyses of genetic changes that could lead to speciation.
Male common shrews (Sorex araneus) with long meiotic chain configurations can be fertile: implications for chromosomal models of speciation.
Two chromosomal races of common shrews (Sorex araneus) were crossed in captivity to generate chain VII-forming complex Robertsonian heterozygotes. Meiosis and gametogenesis were studied in three male hybrids. Regular chain VII configurations were observed at both pachytene and diakinesis/metaphase I, although in many pachytene spreads the chain configuration was incomplete (the basis of this peculiarity is unknown). From metaphase II counts, the frequency of anaphase I nondisjunction in the complex heterozygotes was estimated to be 13%. Germ-cell death in the chain VII-forming complex heterozygotes was 22% greater than it was in controls, but this difference is unlikely to have greatly influenced the capacity of the heterozygotes to sire offspring. Thus, the fecundity of these complex heterozygous common shrews would probably have been only slightly reduced relative to homozygous or simple heterozygous shrews. These results call into question the generality of speciation models based on the presumed sterility of complex heterozygotes.
Hybrid ape offspring of a mating of gibbon and siamang.
The serendipitous mating of a male gibbon, Hylobates moloch, and a female siamang, Symphalangus syndactylus, has produced two female offspring born 1 year apart. The hybrid karyotype of 47 chromosomes comprises the haploid complements of the parental species, 22 for the gibbon and 25 for the siamang. Chromosomal G and C banding comparisons revealed no clear homologies between the parental karyotypes except for the single chromosome in each species containing the nucleolus organizer region. The lack of homology suggests that the structural rearrangement of chromosomes has played a major role in the process of speciation for these lesser apes.
Genetic dissection of hybrid incompatibilities between Drosophila simulans and D. mauritiana. III. Heterogeneous accumulation of hybrid incompatibilities, degree of dominance, and implications for Haldane's rule.
The genetic basis of Haldane's rule was investigated through estimating the accumulation of hybrid incompatibilities between Drosophila simulans and D. mauritiana by means of introgression. The accumulation of hybrid male sterility (HMS) is at least 10 times greater than that of hybrid female sterility (HFS) or hybrid lethality (HL). The degree of dominance for HMS and HL in a pure D. simulans background is estimated as 0.23-0.29 and 0.33-0.39, respectively; that for HL in an F1 background is unlikely to be very small. Evidence obtained here was used to test the Turelli-Orr model of Haldane's rule. Composite causes, especially, faster-male evolution and recessive hybrid incompatibilities, underlie Haldane's rule in heterogametic male taxa such as Drosophila (XY male and XX female). However, if faster-male evolution is driven by sexual selection, it contradicts Haldane's rule for sterility in heterogametic-female taxa such as Lepidoptera (ZW female and ZZ male). The hypothesis of a faster-heterogametic-sex evolution seems to fit the current data best. This hypothesis states that gametogenesis in the heterogametic sex, instead of in males per se, evolves much faster than in the homogametic sex, in part because of sex-ratio selection. This hypothesis not only explains Haldane's rule in a simple way, but also suggests that genomic conflicts play a major role in evolution and speciation.
Speciation of thermotolerant Campylobacter isolates involved in foodborne disease by means of DNA restriction analysis and molecular probes.
The molecular identification of several strains of Campylobacter jejuni and Campylobacter coli involved in foodborne disease was carried out by investigating the restriction profiles of their chromosomal DNA and by DNA/DNA hybridization. Cleavage with EcoRV allowed the visualization of a 3 kb DNA fragment characteristic of C. jejuni, whereas restriction with ClaI allowed the identification of a 9.3 kb DNA fragment, also characteristic of C. jejuni, and a DNA duplet of 9.5-10 kb, specific to C. coli. Restriction analysis with enzyme BglII allowed the visualization of DNA fragments of 3.5, 4, and 6.7 kb, characteristic of C. jejuni. C. jejuni subsp. doylei strains investigated shared a higher genetic homology among themselves-as determined by DNA/DNA hybridization-than with C. jejuni subsp. jejuni. A DNA probe, initially designed by Korolik et al. (Korolik, V.; Coloe, P. J.; Krishnapillai, V. J. Gen. Microbiol. 1988, 134, 521-529), including a DNA fragment encoding an antigenic membrane protein of 31.5 kDa in C. jejuni, when used as probe, allowed the specific identification of all strains of C. jejuni through the detection of strong hybridization signals in two BglII DNA fragments of 2.3 and 2.5 kb, which were not observed in C. coli. Cleavage of chromosomal DNA with BglII-either alone or coupled with probing assays with specific probes-proved to be a valuable tool for the speciation of Campylobacter isolates involved in foodborne disease.