Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra

Abstract. It has long been speculated that Physa acuta , a pulmonate snail widespread and invasive in fresh waters of the old world, may have originated in North America. But the identification of a new-world cognate has been complicated by the confused systematics and taxonomy of the Physidae in America. More than 40 species of physids are currently recognized in the United States, many with variable and overlapping morphology. We have previously established that premating reproductive isolation is negligible among physid snails. Here we report the results from no-choice crosses each involving 2 populations of the widespread American species Physa heterostropha and Physa integra , both with each other and with P. acuta , designed to compare measures of reproductive success between species and between populations within species. Samples of P. acuta were collected from France and Ireland, P. heterostropha from eastern Pennsylvania and South Carolina, and P. integra from southern Indiana and northern Michigan. The 6 intrapopulation controls varied quite significantly in their survival, age at first reproduction, parental fecundity, F₁ viability, and F₁ fertility under our culture conditions. Measures of survival and reproduction in the 6 interpopulation crosses were generally intermediate, but in no case significantly worse than the more poorly performing control. Thus we were unable to detect evidence of reproductive isolation among our 6 populations of snails from 2 continents. All should be referred to the oldest available nomen, P. acuta .     

Evidence for coadaptation in geographic populations of Drosophila bipectinata

In order to test whether there is genetic coadaptation in geographic populations of Drosophila bipectinata with respect to body size, reciprocal crosses were made among five strains derived from ecogeographically different localities in India. Wing length was used as an index of body size, and was measured in all the five strains, and their crosses in F and F₂ generations. The statistical analysis of the data show that there is significant interpopulation variation in body size and in all the crosses, there is an increase in body size in F₁ generation when compared with mid-parent value. Further, there is a decrease in body size in F₂ generation as compared to F₁ in most of the crosses with increased variability. These results provide evidence for genetic coadaptation in geographic populations of D. bipectinata .     

The evolution of F1 postzygotic incompatibilities in birds

Abstract.— We analyzed the rate at which postzygotic incompatibilities accumulate in birds. Our purposes were to assess the role of intrinsic F₁ hybrid infertility and inviability in the speciation process, and to compare rates of loss of fertility and viability between the sexes. Among our sample more than half the crosses between species in the same genus produce fertile hybrids. Complete loss of F₁ hybrid fertility takes on the order of millions of years. Loss of F₁ hybrid viability occurs over longer timescales than fertility: some viable hybrids have been produced between taxa that appear to have been separated for more than 55 my. There is strong support for Haldane's rule, with very few examples where the male has lower fitness than the female. However, in contrast to Drosophila , fertility of the homogametic sex in the F₁ appears to be lost before viability of the heterogametic sex in the F₁. We conclude that the time span of loss of intrinsic hybrid fertility and viability is often, but not always, longer than the time to speciation. Premating isolation is an important mechanism maintaining reproductive isolation in birds. In addition, other factors causing postzygotic reproductive isolation such as ecological causes of hybrid unfitness, reduced mating success of hybrids, and genetic incompatibilities in the F₂s and backcrosses may often be involved in the speciation process.     

Postzygotic isolation between the two European subspecies of the house mouse: estimates from fertility patterns in wild and laboratory-bred hybrids

We assessed the fertility (reproductive success, litter size, testis weight, spermatocyte-to-spermatid ratio) of F₁s and backcrosses between different wild-derived outbred and inbred strains of two mouse subspecies, Mus musculus domesticus and M. m. musculus . A significant proportion of the F₁ females between the outbred crosses did not reproduce, suggesting that female infertility was present. As the spermatocyte-to-spermatid ratio was correlated with testis weight, the latter was used to attribute a sterile vs. fertile phenotype to all males. Segregation proportions in the backcrosses of F₁ females yielded 11 (inbred) to 17% (outbred) sterile males, suggesting the contribution of two to three major genetic factors to hybrid male sterility. Only one direction of cross between the inbred strains produced sterile F₁ males, indicating that one factor was borne by the musculus X-chromosome. No such differences were observed between reciprocal crosses in the outbred strains. The involvement of the X chromosome in male sterility thus could not be assessed, but its contribution appears likely given the limited introgression of X-linked markers through the hybrid zone between the subspecies. However, we observed no sterile phenotypes in wild males from the hybrid zone, although testis weight tended to decrease in the centre of the transect.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 379–393.     

Fertility of roach × bream hybrids, Rutilus rutilus (L.) ×Abramis brama (L.), and their identification

Adult roach, bream and their presumed F₁ hybrid from an Anglian Water reservoir were identified on the basis of morphological and meristic characteristics. The hybrid was clearly intermediate. Four hybrid breeding crosses were induced to spawn by hypophysis. A bream × roach cross (female named first) failed to produce fertile eggs, whereas F₁ hybrid × roach, roach × F₁ hybrid and F₁ hybrid × F₁ hybrid all produced fry. Fertility (defined as survival of eggs to hatching) was high for the F₁ hybrid × roach back-cross (56%) but low for the others (<2%), in comparison to the pure species controls (roach 69%, bream 76%). Progeny from these crosses were reared until anal fin rays could be counted. These counts indicated intermediacy between the parents and back-crossed individuals, and similarity between F₁ hybrids and their F₂ progeny.     

Breeding experiments to distinguish two sibling species of Halicarcinus (Crustacea, Brachyura)

Hybridization experiments and various salinities were used in investigating the status of two Halicarcinus lacustris-like , forms, one of which has no free larval stages (direct-development) and the other of which has normal larval stages (indirect-development).
Hybrids were produced, at least from indirect-development female x direct-development male crosses, but introgression of genetic material was limited to F₂ hybrids from the female parental form. There were no intergrades between the two modes of development in F₁ and F₂ hybrids or as a response to salinity.
These forms are established as sibling species. The name lacustris applies to the direct-development form and the other is a new species, H. paralacustris , which is described. Hybridization in the field is unlikely due to geographical and reproductive isolating mechanisms. Contrary to a previous view, it seems that H. lacustris had a recent origin in Australia and transport by water birds is suggested as the mode of dispersal through south-east Australasia.     

Prospects for exploitation of disease resistance from Hordeum chilense in cultivated cereals

Hordeum chilense is a South American wild barley with high potential for cereal breeding given its high crossability with other members of the Triticeae. In the present paper we consider the resistance of H. chilense to several fungal diseases and the prospects for its transference to cultivated cereals. All H. chilense accessions studied are resistant to the barley, wheat and rye brown rusts, the powdery mildews of wheat, barley, rye and oat, to Septoria leaf blotch, common bunt and to loose smuts, which suggests that H. chilense is a non-host of these diseases. There are also lines resistant to wheat and barley yellow rust, stem rust and to Agropyron leaf rust, as well as lines giving moderate levels of resistance to Septoria glume blotch, tan spot and Fusarium head blight. Some H. chilense lines display pre-appressorial avoidance to brown rust. Lines differ in the degree of haustorium formation by rust and mildew fungi they permit, and in the degree to which a hypersensitive response occurs after haustoria are formed. Unfortunately, resistance of H. chilense to rust fungi is not expressed in tritordeum hybrids, nor in chromosome addition lines in wheat. In tritordeum, H. chilense contributes quantitative resistance to wheat powdery mildew, tan spot and loose smut. The resistance to mildew, expressed as a reduced disease severity, is not associated with macroscopically visible necrosis. Hexaploid tritordeums are immune to Septoria leaf blotch and to common bunt although resistance to both is slightly diluted in octoploid tritordeums. Studies with addition lines in wheat indicate that the resistance of H. chilense to powdery mildew, Septoria leaf blotch and common bunt is of broad genetic basis, conferred by genes present on various chromosomes.     

Population differentiation in an annual legume: genetic architecture

Abstract. The presence or absence of epistasis, or gene interaction, is explicitly assumed in many evolutionary models. Although many empirical studies have documented a role of epistasis in population divergence under laboratory conditions, there have been very few attempts at quantifying epistasis in the native environment where natural selection is expected to act. In addition, we have little understanding of the frequency with which epistasis contributes to the evolution of natural populations. In this study we used a quantitative genetic design to quantify the contribution of epistasis to population divergence for fitness components of a native annual legume, Chamaecrista fasciculata . The design incorporated the contrast of performance of F₂ and F₃ segregating progeny of 18 interpopulation crosses with the F₁ and their parents. Crosses were conducted between populations from 100 m to 2000 km apart. All generations were grown for two seasons in the natural environment of one of the parents. The F₁ often outperformed the parents. This F₁ heterosis reveals population structure and suggests that drift is a major contributor to population differentiation. The F₂ generation demonstrated that combining genes from different populations can sometimes have unexpected positive effects. However, the F₃ performance indicated that combining genes from different populations decreased vigor beyond that due to the expected loss of heterozygosity. Combined with previous data, our results suggest that both selection and drift contribute to population differentiation that is based on epistatic genetic divergence. Because only the F₃ consistently expressed hybrid breakdown, we conclude that the epistasis documented in our study reflects interactions among linked loci.     

Resurrection of Aedes (Halaedes) ashworthi Edwards 1921: Morphological Characterisation and Separation from its Sibling Aedes (Halaedes) australis (Erichson) 1842 (Diptera: Culicidae)

The result of population crosses of members of the Ae. australis complex and the examination of the male genitalia of these populations have shown that Ae. ashworthi Edwards 1921 should be reinstated as a valid species. the parent females laid viable eggs when cross mated to males of different species, but the resulting F₁ females and males were largely sterile. When F₁ females were blood-fed and examined 3–8 d later, usually fewer than 10 ovarioles per ovary developed recognisable follicles and none of these formed eggs. F₁ males in nearly all cases possessed half-size testes, and there was no spermatid development in the reduced testes. A few males had normal-sized testes, but the spermatids were mostly tailless. the male of Ae. ashworthi can be separated from Ae. australis by the shape of the gonocoxite and the length of the gonostylus of the genitalia. the larvae of both species are identical, and the females differ only in the colour of some scale patches and the integument, which may prove to be too variable to be useful when more populations are compared. the morphological similarity of the two mosquitoes as larvae and adults supports the proposal that they should be recognised as sibling species, with Ae. ashworthi as the derived member of the Ae. australis complex. It is clear from morphological evidence, the viability of the F₁ hybrids and the unusual habitat occupied by both species, that they have diverged recently.     

Anomalous Serotypes in Tetrahymena

SYNOPSIS. Novel serotypes appeared among the F₂ progeny of a single cross involving alleles at the H locus. Two of the clones manifesting aberrant types have been subjected to breeding analysis. The aberrant phenotypes disappear at conjugation and have not been recovered in either F₁ or F₂. Comparable types have not been found again in large scale crosses involving a variety of different H alleles. The basis for the unusual types remains obscure.     

Cytogenetics of Hordeum chilense: current status and considerations with reference to breeding

Hordeum chilense Roem. et Schult. has a number of characteristics interesting for breeding: high crossability with other Triticeae, resistance to biotic and abiotic stresses and high variability for quality traits such as endosperm storage proteins or carotenoid content. xTritordeum, the amphiploids between H. chilense and different Triticum spp, are bridge species which facilitate the transfer of traits from H. chilense to wheat or triticale. The chromosome pairing between H. chilense and wheat chromosomes is very low (if existing) even in the absence of the action of the Ph1 gene. Nevertheless, translocation between H. chilense and wheat chromosomes has been observed frequently in genomic combinations where univalents of both species are present and therefore a method is available for using H. chilense in wheat or triticale breeding. Hybrids and amphiploids with other crop species of the Triticeae, such as rye or barley, have also been obtained, although to date the production of stable introgression stocks has not been completed. The technique of chromosome painting, using both high- and low-repeated DNA sequences in combination with genomic in situ hybridization have been used as effective methods for basic cytogenetic research in H. chilense, allowing analysis of genome evolution, and monitoring H. chilense chromosomes in interspecific hybridization breeding programs.     

Identification of intergenomic translocations involving wheat,Hordeum vulgare and Hordeum chilense chromosomes by FISH

Intergenomic translocations between wheat, Hordeum chilense and Hordeum vulgare have been obtained in tritordeum background. Advanced lines from the crosses between three disomic chromosome addition lines for chromosome 2Hv, 3Hv, and 4Hv of barley (Hordeum vulgare) in Triticum aestivum cv. Chinese Spring (CS) and hexaploid tritordeum (2n = 6x = 42, AABBHchHch) were analyzed. Multicolor FISH using both genomic DNA from H. chilense and H. vulgare were used to establish the presence and numbers of H. vulgare introgressions into tritordeum. Interspecific H. vulgare/H. chilense and intergeneric wheat/H. vulgare and wheat/H. chilense translocations were identified. Frequencies of plants containing different kinds of intergenomic translocations between chromosome arms are presented. These lines can be useful for introgressing into tritordeum characters of interest from H. vulgare.     

Increasing tolerance to Cry1Ac cotton from cotton bollworm, Helicoverpa armigera, was confirmed in Bt cotton farming area of China

Abstract.  1. Changes in the frequency of Cry1Ac resistance genes and shifts in tolerance of cotton bollworm, Helicoverpa armigera , to the Cry1Ac toxin were assessed using bioassays of F₁ and F₂ offspring of isofemale lines from Anci County of Hebei Province (a multiple-crop system including corn, soybean, peanut, and Bt cotton) and Xiajin County of Shandong Province (an intensive Bt cotton planting area) in Northern China during 2002–2005.
2. A conservative analysis of the overall results indicated that there was a small increase in the frequency of major, non-recessive resistance genes over time.
3. The relative average development ratings [RADR – growth rate of a line on a Bt diet in proportion to the growth rate on a non-Bt (NBT) diet] of the bollworm larvae in F₁ tests increased significantly from year to year, indicating a gradual trend towards higher tolerance to Cry1Ac in the field populations.
4. There were also significant positive correlations between RADR of the lines in the F₁ generation and the RADR of their F₂ offspring, indicating that the tolerance was genetically based.
5. Quantitative genetic simulation analysis showed that resistance of H . armigera to Bt cotton in Xiajin could evolve to a high level in 11–15 years if no effective resistance management measures are carried out.     

Genetic variability and heritability of drought-induced abscisic acid accumulation in spring wheat

Abstract. Twenty cultivars of spring wheat were examined for variation in abscisic acid (ABA) accumulation following partial dehydration of excised leaves. A 3-fold range of ABA concentration was obtained.
A cross between two cultivars which differed in drought-induced ABA accumulation was used to study the heritability of ABA accumulation and to develop lines differing in their capacity to accumulate ABA. Broad sense heritability was 0–32 between the F₂ and F₃ generations and 0–70 between the F₃ and F₄ generations. Apparent homozygosity for ABA accumulation was achieved in several selections at the F₄. The possible significance for drought resistance of differences in capacity to accumulate ABA is discussed.     

Interspecific crosses inHordeum (Poaceae)

A crossing programme including 30 species and 40 cytotypes within the genusHordeum was undertaken. Viable hybrids were obtained in 302 combinations, 15 of which were intraspecific. Differences in seed set and in germination were observed in crosses between different groups of species. Obtaining crosses between different taxonomic groups was generally more difficult when diploid material was used. Some species, e.g.,H. lechleri, H. jubatum, andH. brachyantherum showed a higher crossability than others. The chromosome numbers of the hybrids were usually those expected from the parental numbers but aneuploid series around the expected numbers were rather frequent. Three cases of unreduced gametes were found. Selective chromosome elimination was restricted to combinations including eitherH. vulgare orH. bulbosum.—Despite a very diverse morphology, all South American diploid species together with the two North American diploidsH. intercedens andH. pusillum appear to be closely related. The hexaploid American speciesH. procerum, H. lechleri, andH. arizonicum are also related. The two North American tetraploid speciesH. jubatum andH. brachyantherum sometimes form semifertile hybrids. The Asiatic speciesH. roshevitzii appears to be related to both North and South American taxa.     

Chloroplast ultrastructure in two Crassulacean species and an F1 hybrid with differing biomass δ13C values

Abstract. The chloroplasts of two species of the Crassulaceae and their F₁ hybrid were compared by electron microscopy. The two species had contrasting leaf tissue δ¹³C values of −25°/_∞ ( Sedum greggii ) and −13°/_∞ ( Cremnophila linguifolia ), and the F₁ hybrid had a value of − 18°/_∞. S. greggii had a mean of 8.9 thylakoids per granum in contrast to C. linguifolia which had a mean of only 3.8 thylakoids per granum. The F₁ hybrid had a mean of 6.4 thylakoids per granum. Crystaloids were observed in S. greggii and the hybrid but not in C. linguifolia     

Artificial hybridization within Saxifraga pentadactylis (Saxifragaceae)

Saxifraga pentadactylis subsp. almanzorii , an endemic to the subalpine nucleus of Sierra de Gredos (central Spain), differs from its closest relative, subsp. willkommiana , by its less showy petals. An artificial crossing program was carried out in order to assess the degree of reproductive isolation between the subspecies. To facilitate interpretation of the results, the program was extended to 10 other interspecific hybrid combinations within sect. Saxifraga . All the data gathered are congruent with the occurrence of two evolutionary scenarios. Interspecific crossings, rendering moderate to high seed-set (in obtaining the F₁), and vigorous but relatively sterile F₁ offspring, reveal reproductive barriers at the level of the F₁ fertility, probably originated as a byproduct of divergent evolution. In contrast, intraspecific crossings within S. pentadactylis resulted in seed-set values lower than expected (in obtaining the F₁), in a majority of weak non-viable F₁ offspring but also in a few fertile F₁ hybrid specimens which were able to originate F₂ offspring. This second pattern reveals reproductive barriers at the level of the F₁ vitality, probably arisen in a quite abrupt fashion. The lower P/O for subsp. almanzorii as compared to subsp. willkommiana , together with the rest of the evidence suggest that the reproductive barriers between them might be the product of active selection against hybridization achieved by incrementing the levels of autogamy in the former.     

QTL mapping provides evidence for lack of association of the avoidance of leaf rust in Hordeum chilense with stomata density

In cereals, rust fungi are among the most harmful pathogens. Breeders usually rely on short-lived hypersensitivity resistance. As an alternative, "avoidance" may be a more durable defence mechanism to protect plants to rust fungi. In Hordeum chilense avoidance is based on extensive wax covering of stomata, which interferes with the induction of appressorium formation by the rust fungi. High avoidance levels are associated with a higher stoma density on the abaxial leaf epidermis. The avoidance level was assessed as the percentage of germ tube/stoma encounters that did not result in appressorium differentiation by Puccinia hordei, the barley leaf rust fungus. One hundred F(2) individuals from the cross between two H. chilense accessions with contrasting levels of avoidance showed a continuous distribution for avoidance of the rust fungus and for stoma density, indicating quantitative inheritance of the traits. No significant correlation was found between avoidance and stoma density in the segregating F(2) population. In order to map quantitative trait loci (QTLs) for both traits, an improved molecular marker linkage map was constructed, based on the F(2) population. The resulting linkage map spanned 620 cM and featured a total of 437 AFLP markers, thirteen RFLPs, four SCARs, nine SSRs, one STS and two seed storage protein markers. It consisted of seven long and two shorter linkage groups, and was estimated to cover 81% of the H. chilense genome. Restricted multiple interval mapping identified two QTLs for avoidance and three QTLs for stoma density in the abaxial leaf surface. The QTLs for avoidance were mapped on chromosome 3 and 5; those for stoma density on chromosomes 1, 3 and 7. Only the two QTLs regions located on chromosome 3 (one for avoidance and the other for stoma density) overlapped. The wild barley H. chilense has a high crossability with other members of the Triticeae tribe. The knowledge on the location of the QTLs responsible for the avoidance trait is a prerequisite to transfer this favourable agronomic trait from H. chilense to cultivated cereal genomes.     

Natural hybridization between endangered and introduced species of Pseudorasbora, with their genetic relationships and characteristics inferred from allozyme analyses

From 25 populations of Pseudorasbora in Japan, polymorphism at 22 allozyme loci indicated that the level of genetic differentiation between Pseudorasbora parva and Pseudorasbora pumila ( D  = 0·421–0·517) was greater than that between the two subspecies of P. pumila , P. pumila pumila and P. pumila subsp. ( D  = 0·164), consistent with morphological differences. While P. parva displayed genetic variation ( H  = 0·003–0·100) similar to other freshwater fishes, P. pumila pumila and P. pumila subsp. populations showed no genetic variation. In five of the 15 populations collected from the contact zone between P. parva and P. pumila pumila , hybrids were detected by allozyme analyses. All hybrids were presumed to represent the F₁ generation, because they were heterozygous at all 12 loci diagnostic between P. parva and P. pumila pumila . Although four populations were characterized by high frequencies of F₁ hybrids ( c . 40%), only one of the parental species was observed in each case. The results indicated that the two species have hybridized easily under natural conditions, but cannot coexist in the long-term. It is suggested that continued invasion of P. parva would hasten the extinction of P. pumila pumila .     

INHERITANCE OF RESISTANCE TO CYHALOTHRIN IN THE HOUSEFLY (DIPTERA: MUSCIDAE)

Abstract The genetic inheritance of resistance to cyhalothrin in housefly, Musca domstica (L) was investigated.
Reciprocal crosses between susceptible (S) and resistant (R) strains were used to determine the characteristics of resistance. Analysis of probit line from the F₁ generation and F₂ generation obtained by inbreeding the F₁ hybrids indicated that cyhalothrin resistance was controlled by more than one factors and degree of resistance dominance to cyhalothrin was -0.10, indicating cyhalothrin resistance is conferred by incompletely recessive gene(s). The realized heritability of resistance to cyhalothrin cyhalothrin calculated from data collected routinely from laboratory selection was 0.12.