Patterns of postzygotic isolation in Lepidoptera

I present patterns characterizing the evolution of intrinsic postzygotic isolation in Lepidoptera by analyzing data from the literature on genetic distance, strength of hybrid sterility and inviability, biogeography, and natural hybridization. Using genetic distance as a proxy for time, I investigate the time-course of the evolution of postzygotic isolation and the waiting times to particular hybrid fitness problems. The results show that postzygotic isolation increases gradually as species diverge, but that hybrid sterility evolves faster than hybrid inviability. The overwhelming preponderance of female-specific hybrid problems in Lepidoptera shows that Haldane's rule (the preferential sterility or inviability of the heterogametic sex) is well obeyed. Together the rates and patterns characterizing the accumulation of postzygotic isolation allow several tests of the composite theory of Haldane's rule. Interestingly, comparing these data with those from Drosophila reveals that Haldane's rule for sterility evolves as fast (if not faster) in Lepidoptera. Finally, I show that a substantial fraction of sympatric species hybridizes in nature and that the majority of these suffer some level of hybrid sterility or inviability.     

Genetic diversity and genetic differentiation in Daphnia metapopulations with subpopulations of known age

If colonization of empty habitat patches causes genetic bottlenecks, freshly founded, young populations should be genetically less diverse than older ones that may have experienced successive rounds of immigration. This can be studied in metapopulations with subpopulations of known age. We studied allozyme variation in metapopulations of two species of water fleas (Daphnia) in the skerry archipelago of southern Finland. These populations have been monitored since 1982. Screening 49 populations of D. longispina and 77 populations of D. magna, separated by distances of 1.5-2180 m, we found that local genetic diversity increased with population age whereas pairwise differentiation among pools decreased with population age. These patterns persisted even after controlling for several potentially confounding ecological variables, indicating that extinction and recolonization dynamics decrease local genetic diversity and increase genetic differentiation in these metapopulations by causing genetic bottlenecks during colonization. We suggest that the effect of these bottlenecks may be twofold, namely decreasing genetic diversity by random sampling and leading to population-wide inbreeding. Subsequent immigration then may not only introduce new genetic material, but also lead to the production of noninbred hybrids, selection for which may cause immigrant alleles to increase in frequency, thus leading to increased genetic diversity in older populations.     

Geographical variation in postzygotic isolation and its genetic basis within and between two Mimulus species

The aim of this study is to investigate the evolution of intrinsic postzygotic isolation within and between populations of Mimulus guttatus and Mimulus nasutus. We made 17 intraspecific and interspecific crosses, across a wide geographical scale. We examined the seed germination success and pollen fertility of reciprocal F₁ and F₂ hybrids and their pure-species parents, and used biometrical genetic tests to distinguish among alternative models of inheritance. Hybrid seed inviability was sporadic in both interspecific and intraspecific crosses. For several crosses, Dobzhansky–Muller incompatibilities involving nuclear genes were implicated, while two interspecific crosses revealed evidence of cytonuclear interactions. Reduced hybrid pollen fertility was found to be greatly influenced by Dobzhansky–Muller incompatibilities in five out of six intraspecific crosses and nine out of 11 interspecific crosses. Cytonuclear incompatibilities reduced hybrid fitness in only one intraspecific and one interspecific cross. This study suggests that intrinsic postzygotic isolation is common in hybrids between these Mimulus species, yet the particular hybrid incompatibilities responsible for effecting this isolation differ among the populations tested. Hence, we conclude that they evolve and spread only at the local scale.     

山楂叶螨在苹果不同品种上的生长发育及繁殖

室内试验结果表明 ,山楂叶螨TetrancychusvienensisZacher在苹果不同品种上的生长发育和繁殖有较大的差异。其中在红星上存活率高 ,发育速度快 ,生殖力强 ;而在秦冠上存活率低、发育速度慢、生殖力低。对山楂叶螨种群年结龄构的分析表明 ,卵在种群中所占比例较高 ,而成虫所占比例甚低 ,建议在对其进行防治时选用杀卵力强的药剂     

Genetics of postzygotic reproductive isolation in plants

Postzygotic reproductive isolation, based on negative interactions of genes, is a key aspect of divergent speciation in plants and animals. The effect of the interaction manifests as a drastic reduction in fitness of hybrids of the first of subsequent generations, which is expressed as hybrid lethality, weakness or sterility. Both simple genetic control of genetic incompatibility, which includes interallelic negative complementation or epistatic interactions of a limited number of genes, and more complex control, based on epistatic interactions of many genes, have been described in plants. It is thought that genes for reproductive isolation are nonuniformly distributed over the genome and are related to differential adaptation. The mosaic organization of the genomes in this respect provides restoration of their structural and functional integrity upon interspecies hybridization in natural conditions. Many cultured and wild plant species, in contrast to animals, were found to be polymorphic at genes controlling interspecies incompatibility. This fact facilitates genetic analysis of incompatibility and broadens perspectives in studying the structure, functions, and molecular evolution of the genes controlling postzygotic reproductive isolation, in view of the possible leading role of these genes in adaptive speciation.     

Genetics of postzygotic isolation and Haldane's rule in haplodiploids

The process of speciation has puzzled scientists for decades, but only recently they have they been able to reveal the genetic basis of reproductive isolation. Much emphasis has been on Haldane's rule, the observation that the heterogametic sex often suffers more from hybridization than the homogametic sex. Most research on Haldane's rule has focused on diploid organisms with chromosomal sex determination. We argue that species lacking chromosomal sex determination, such as haplodiploids, also follow Haldane's rule and thus should be included in the definition of this rule. We provide evidence for Haldane's rule in Nasonia wasps and describe how haplodiploids can be used to test the different theories that have been proposed to explain Haldane's rule. We discuss how the faster-male and faster-X theories can shape speciation differently in haplodiploids compared to diploids.     

Hybridization and postzygotic isolation patterns in pigeons and doves

Abstract.— The study of the patterns of reproductive isolation in relation to species divergence is critical for the understanding of the process of speciation. Comparative analyses of this kind were previously conducted in Drosophila , Lepidoptera, frogs, ducks, and birds in general. In the present study, we used information from the literature to analyze hybrid inviability in relation to species divergence in pigeons and doves. Four main patterns arose from this analysis: (1) as in the other groups studied, F₁ hybrid inviability gradually increases as species diverge, the time needed to reach total inviability being higher in birds than in the other groups; (2) as expected, the presence of geographic overlap does not influence the evolution of postzygotic isolation; (3) the percentage of unhatched eggs does not differ between hybrids of the first generation and the backcrosses, but it increases in the second hybrid generation; and (4) pigeons and doves follow Haldane's rule, as found in the other groups studied so far. The similarity between the results of this and previous studies contributes to the growing evidence suggesting that the patterns of the evolution of postzygotic isolation, and the process of speciation in general, are shared among animal groups.     

Photoperiodic time measurement in spider mites

Summary A model has been developed which describes in mathematical terms the incidence of diapause in the spider miteTetranychus urticae under different photoperiodic regimens. The model has been derived from Beck's (1974a, b, 1975, 1976a) Dual System Theory of photoperiodic time measurement, by means of a number of essential alterations and modifications. The spider mite's model is composed of two hour-glass timers, one of which starts at lights-off and measures the length of the night, whereas the other is initiated both by the onset of the first hour-glass timer and by lights-on. This second hour-glass defines the time at which the first hour-glass is read off, the state of the first hour-glass at this particular time being decisive for the developmental alternative (diapause or nondiapause) to be determined. The model may be classified as a form of internal coincidence according to the terminology of Pittendrigh (1972), since it is based on the interaction of two internal systems rather than on the coincidence of light with a particular light-sensitive phase of the timing mechanism, as in the case of external coincidence (cf. Saunders, 1978). Good agreement is attained between diapause incidences predicted by this model and incidences observed in spider mites, both in the diapause induction response curve and in asymmetrical skeleton photoperiods.The authors are grateful to Dr. S.D. Beck for the program printout of the Dual System Theory put at their disposal. It is also a pleasure to thank Miss C. van Ruth and Mr. G. van de Berg for technical assistance, and Mr. H. Bos for drawing the figures. The investigations were supported by a grant to the first author from the Netherlands Organization for the Advancement of Pure Research (ZWO).     

Reproductive isolation due to prezygotic isolation and postzygotic cytoplasmic incompatibility in parasitoid wasps

The reproductive barriers that prevent gene flow between closely related species are a major topic in evolutionary research. Insect clades with parasitoid lifestyle are among the most species‐rich insects and new species are constantly described, indicating that speciation occurs frequently in this group. However, there are only very few studies on speciation in parasitoids. We studied reproductive barriers in two lineages of Lariophagus distinguendus (Chalcidoidea: Hymenoptera), a parasitoid wasp of pest beetle larvae that occur in human environments. One of the two lineages occurs in households preferably attacking larvae of the drugstore beetle Stegobium paniceum (“DB‐lineage”), the other in grain stores with larvae of the granary weevil Sitophilus granarius as main host (“GW‐lineage”). Between two populations of the DB‐lineage, we identified slight sexual isolation as intraspecific barrier. Between populations from both lineages, we found almost complete sexual isolation caused by female mate choice, and postzygotic isolation, which is partially caused by cytoplasmic incompatibility induced by so far undescribed endosymbionts which are not Wolbachia or Cardinium. Because separation between the two lineages is almost complete, they should be considered as separate species according to the biological species concept. This demonstrates that cryptic species within parasitoid Hymenoptera also occur in Central Europe in close contact to humans.     

Temperature-mediated stability of the interaction between spider mites and predatory mites in orchards

The nonlinear behavior of the Holling-Tanner predatory-prey differential equation system, employed by R.M. May to illustrate the apparent robustness of Kolmogorov’s Theorem when applied to such exploitation systems, is re-examined by means of the numerical bifurcation code AUTO 86 with model parameters chosen appropriately for a temperature-dependent mite interaction on fruit trees. The most significant result of this analysis is that there exists a temperature range wherein multiple stable states can occur, in direct violation of May’s interpretation of this system’s satisfaction of Kolmogorov’s Theorem: namely, that linear stability predictions have global consequences. In particular these stable states consist of a focus (spiral point) and a limit cycle separated from each other in the phase plane by an unstable limit cycle, all of which are associated with the single community equilibrium point of the system. The ecological implications of such metastability, hysteresis, and threshold behavior for the occurrence of outbreaks, the persistence of oscillations, the resiliency of the system, and the biological control of mite populations are discussed.     

Dominance, epistasis and the genetics of postzygotic isolation

The sterility and inviability of species hybrids can be explained by between-locus "Dobzhansky-Muller" incompatibilities: alleles that are fit on their "normal" genetic backgrounds sometimes lower fitness when brought together in hybrids. We present a model of two-locus incompatibilities that distinguishes among three types of hybrid interactions: those between heterozygous loci (H(0)), those between a heterozygous and a homozygous (or hemizygous) locus (H(1)), and those between homozygous loci (H(2)). We predict the relative fitnesses of hybrid genotypes by calculating the expected numbers of each type of incompatibility. We use this model to study Haldane's rule and the large effect of X chromosomes on postzygotic isolation. We show that the severity of H(0) vs. H(1) incompatibilities is key to understanding Haldane's rule, while the severity of H(1) vs. H(2) incompatibilities must also be considered to explain large X effects. Large X effects are not inevitable in backcross analyses but rather-like Haldane's rule-may often reflect the recessivity of alleles causing postzygotic isolation. We also consider incompatibilities involving the Y (or W) chromosome and maternal effects. Such incompatibilities are common in Drosophila species crosses, and their consequences in male- vs. female-heterogametic taxa may explain the pattern of exceptions to Haldane's rule.     

Rapid evolution of postzygotic reproductive isolation in stalk-eyed flies

We test the relative rates of evolution of pre- and postzygotic reproductive isolation using eight populations of the sexually dimorphic stalk-eyed flies Cyrtodiopsis dalmanni and C. whitei. Flies from these populations exhibit few morphological differences yet experience strong sexual selection on male eyestalks. To measure reproductive isolation we housed one male and three female flies from within and between these populations in replicate cages and then recorded mating behavior, sperm transfer, progeny production, and hybrid fertility. Using a phylogeny based on partial sequences of two mitochondrial genes, we found that premating isolation, postmating isolation prior to hybrid eclosion, and female hybrid sterility evolve gradually with respect to mitochondrial DNA sequence divergence. In contrast, male hybrid sterility evolves much more rapidly-at least twice as fast as any other form of reproductive isolation. Hybrid sterility, therefore, obeys Haldane's rule. Although some brood sex ratios were female biased, average brood sex ratio did not covary with genetic distance, as would be expected if hybrid inviability obeyed Haldane's rule. The likelihood that forces including sexual selection and intra- and intergenomic conflict may have contributed to these patterns is discussed.     

Impact of ancestral populations on postzygotic isolation in allopatric speciation

Postzygotic isolation evolves due to an accumulation of substitutions (potentially deleterious alleles in hybrids) in populations that have become geographically isolated. These potentially deleterious alleles might also be maintained in ancestral populations before geographic isolation. We used an individual-based model to examine the effect of the genetic state of an ancestral population on the evolution of postzygotic isolation after geographic isolation of a population. The results showed that the number of loci at which degenerative alleles are fixed in an ancestral population at equilibrium significantly affects the evolutionary rates of postzygotic isolation between descendant allopatric populations. Our results suggest that: (1) a severe decrease in population size (e.g., less than ten individuals) is not necessary for the rapid evolution of postzygotic isolation (e.g., <10,000 generation); (2) rapid speciation can occur when there is a large difference in the equilibrium number of accumulated degenerative alleles between ancestral and descendant populations; and (3) in an ancestral population maintained at a small effective population size for a long period of time, postzygotic isolation rarely evolves if back mutations that restore the function of degenerative alleles are limited.     

A molecular‐genetic understanding of diapause in spider mites: current knowledge and future directions

During unfavourable conditions, many arthropods have the ability to enter into diapause and synchronize their development and reproduction to seasonal patterns. Diapause or winter hibernation in insects and mites is set off by a number of cues, with photoperiod being the most well‐defined and strongest signal. This review focuses on the current knowledge of ‘‐omics’ data and the genetics of diapause in the two‐spotted spider mite Tetranychus urticae, a member of the family Tetranychidae (Arthropoda: Chelicerata: Arachnida: Acari). This species is a serious polyphagous pest and females undergo a reproductive facultative diapause when immature stages are exposed to long nights. Winter hibernation induces different physiological processes characterized by a metabolic suppression, different energy use, increased stress tolerance and the production of cryoprotectants, all initiated by a complex signal transduction pathway. Keto‐carotenoids are known to cause the deeply orange colour typical for diapausing females. Furthermore, research with colour mutants of T. urticae has shown the need for carotenoids with respect to the induction of diapause, even though the molecular‐genetic mechanisms underlying these colour phenotypes are still unknown. In addition, marked latitudinal variation in diapause incidence among populations has been observed in nature, with modes of inheritance ranging from recessive to dominant, as well as monogenic to polygenic. We end by highlighting the emerging opportunities for functional studies that aim to unravel the complex factors underlying diapause in spider mites.     

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.     

Karyotypes and sex-determination in spider mites (Tetranychidae)

The chromosomes of 13 species of spider mites (Tetranychidae) are determined using the aceto-orcein squash technique in order to establish the haplo-diploid sex-determination. 12 species showed the existence of haploid and diploid eggs:Neotetranychus rubi (Trägårdh) with 7 and 14 chromosomes;Eurytetranychus buxi (Garman) with 5 and 10;Bryobia sarothamni (Geijskes),Eotetranychus tiliarium (Joh. Hermann) andE. carpini (Oudemans) with 4 and 8;Panonychus ulmi (Koch),Schizotetranychus schizopus (Zacher),Oligonychus ununguis (Jacobi).Tetranychus hydrangeae Pritch. & Baker, T.pacificus McGregor,T. urticae Koch andT. cinnabarinus (Boisduval) with 3 and 6 chromosomes.The progeny of virgin females in 7 different species consisted of eggs with the haploid complement. One species was shown to be thelytokous, viz.Tetranycopsis horridus (Canestrini & Franzago), having a diploid number of 4 chromosomes. Some eggs of an inbred line ofT. urticae showed a patchwork quilt of odd polyploidy in embryonic tissue.     

捕食螨在我国农林害螨生物防治中的应用

捕食螨是农林生产中极具价值的生防作用物。文章对捕食螨在我国农林害螨生物防治中的应用作一综述,内容包括:与释放应用相关的研究(引进与风险评估,饲养、贮存与运输,食性与释放,捕食与繁殖,定殖与扩散,外界影响因子)、几种主要捕食螨(智利小植绥螨Phytoseiulus persimilis、西方盲走螨Typhlodromus occidentalis、伪钝绥螨Amblyseius fallacis、胡瓜钝绥螨A.cucumeris)的应用、释放应用前景与展望等,旨在提高我国的捕食螨研究与应用水平。