Contemporary hybrid speciation in sculpins (Cottus spp.)

Natural hybridization between closely related taxa is frequent in many organismal groups, yet it has long been perceived as a force preventing diversification and speciation, especially so in animals. In recent years, growing evidence in favour of hybridization facilitating adaptive divergence has accumulated ( Mallet 2007 ; Mavárez & Linares 2008 ; Nolte & Tautz 2010 ). Homoploid hybrid speciation (the formation of hybrid lineages without changes in chromosome number) occurs when distinct species come into contact, hybridize, and at least in part of their range, produce hybrid swarms. If the hybrid genotypes can then colonize areas of the adaptive landscape inaccessible to ancestral species, they may eventually form new distinct lineages, reproductively isolated from their ancestors. Invasive sculpins (Cottus sp.) are one of a few good examples of homoploid hybrid speciation in animals. In this issue, Stemshorn et al. (2011) identified three distinct hybrid lineages, which have emerged out of a secondary contact situation of Cottus rhenanus and Cottus perifretum. Hybrids have recently invaded large river habitats unsuitable to ancestral species. Through the use of genetic mapping, the authors established that contrary to expectations, chromosomal rearrangements were not apparent in the hybrid lineages. In addition, different population genetic models were tested and the results suggest that contemporary gene flow from ancestral species represents an important component of the system. As such, recent and ongoing hybridization appears to be promoting the appearance of phenotypes adapted to novel environments. The examination of partially isolated lineages such as invasive hybrid sculpins should permit to identify early adaptive genetic changes before they become confounded by differences arising once speciation is complete.     

Holarctic phylogeography of an asexual species complex – II. Allozymic variation and clonal structure in Arctic Daphnia

Previous mitochondrial (mt)DNA sequencing and restriction fragment length polymorphism (RFLP) studies have shown that the Holarctic Daphnia pulex complex is divisible into two major groups (pulicaria and tenebrosa) that exhibit distinct phylogeographic patterns. Here we examine allozymic variation at six polymorphic enzyme loci to ascertain clonal structure and clonal distribution patterns within each group. Specimens were collected from a total of 850 populations encompassing the Arctic. A significant negative relationship (Mantel test) between similarity of regional clonal arrays and geographical distance was observed. A small fraction of clones in each group was widespread (in the order of 1000s of kilometres). However, most clones were restricted to single regions, and were often found only in a single population. These data indicate that the population genetic structure is highly fragmented in this complex, but the potential for long-distance passive dispersal exists. Further, ‘hot spots’ of high clonal richness and diversity were found in each group, which is concordant with earlier work. In addition, ≈ 20% of pulicaria group clones possess nuclear genes from tenebrosa, while approximately 10% of tenebrosa group clones harbour pulicaria nuclear genes. These data indicate nuclear introgression between the two groups, which was found to be prominent in a broad zone of secondary contact encompassing parts of northwestern Russia, northern Fennoscandia, Svalbard, and extending into the high eastern Canadian Arctic.     

Geographical patterns of allozymic variation in three species of intertidal sculpins

Synopsis We examined spatial patterns of allozyme variation in three species of intertidal cottids to evaluate how much gene flow occurs between geographically separate populations (most likely via passive dispersal of planktonic larvae by currents). Our results from the analysis of geographical patterns of allele frequencies and, to some extent, from comparison of allele frequencies between newly recruited young and older resident fish are consistent with the notion that sufficient gene flow occurs in these fishes to prevent significant genetic isolation of populations. From these results, we visualize evolutionary changes in populations of these species as occurring most likely over a broad geographic scale, affecting spatially separated but genetically linked populations concurrently, rather than operating independently in individual, genetically isolated populations.     

Genetic variability, population size and isolation of distinct populations in the freshwater fish Cottus gobio L.

Analysis of allozyme data of the European freshwater fish Cottus gobio showed marked genetic differentiation across drainage basins in northeastern Bavaria, which points to the existence of at least two cryptic taxa. Genetic variability within populations differed significantly between these two taxa, which could be due to historical (bottlenecks) or ecological reasons (population size). To distinguish between these two hypotheses we sampled 12 distinct populations from Rhine, Elbe and Danube drainages. Using allozyme data we examined the influence of population size and isolation on genetic variability within populations. We used spatial extent of populations (patch size) as a measure for population size. To estimate isolation we calculated a compound measure which took into account patch size and distance to all neighbouring populations. Both patch size and isolation were highly correlated with genetic variability, explaining ≈95% of the variance of genetic variability within populations. Furthermore, analysis of covariance suggests that the difference in genetic variability between taxa may be explained by differences in population size.     

Troglomorphic Sculpins of the Cottus Carolinae Species Group in Perry County, Missouri: Distribution, External Morphology, and Conservation Status

The existence of cavernicolous sculpin (here allocated to Cottus carolinae, banded sculpin, and referred to as grotto sculpin), in the karst regions of Perry County, Missouri, first came to our attention in 1991. Examination of 35 caves in Missouri, 96 in Illinois, 17 in Tennessee, two in Indiana, and 11 in Arkansas revealed that banded sculpin are common in cave habitats; however, grotto sculpin are limited to two karst areas of Perry County, Missouri, where they are known from only six cave systems. These caves and their streams are extensive and apparently provide a unique habitat compared to other karst systems; this may be a critical factor in the present restricted distribution of the grotto sculpin. Grotto sculpin occupy pools and riffles of cave streams, and occur over a variety of substrates, from sediment to breakdown. Density estimates in Mystery and Running Bull caves were 0.29 and 0.63 individuals m^-2, respectively. Grotto sculpin have small eyes (1–6% SL vs. 6–10% SL in epigean samples), significantly reduced pigmentation (including nearly complete loss of dorsal saddles), a reduction in pelvic fin ray number (from 4+4 elements to often 4+3 , or 3+3), and enlarged cephalic lateralis pores (e.g., mandibular pores of cavernicolous samples are 2–3 times those of epigean stream samples). Multivariate analyses of body shape revealed statistically significant separation of epigean and hypogean samples, with eye size highly variable, but smallest in the Running Bull Cave population. We interpret these results as representative of losses associated with long-term cave habitation. Caves of Perry County provide ample habitat for grotto sculpin, but because the caves are located downgradient of the city of Perryville and an intensively farmed landscape, point and non-point source pollution threaten their continued existence. Escape of farm-pond fishes through the extensive sinkhole network in Perry County has increased potential predation pressure on grotto sculpin by channel catfish, Ictalurus punctatus, and other species normally excluded from cave environments.     

Genetic variation and differentiation of the two river sculpins,Cottus nozawae andC. amblystomopsis,deduced from allozyme and restriction enzyme-digested mtDNA fragment length polymorphism analyses

Genetic differentiation of the two sibling species,Cottus nozawae andC. amblystomopsis, from the northern part of Japan (Hokkaido Island and the Tohoku District) was investigated using allozyme variations and restriction fragment length polymorphisms of mitochondrial DNA. Although the two species are morphologically very similar, previously being thought to be a single species, they have different life-cycles;C. nozawae has a fluvial life-cycle with a small number of large-sized eggs, whereasC. amblystomopsis is an amphidromous species with a large number of small-sized eggs. Four populations ofC. amblystomopsis from Hokkaido Island and 24 populations ofC. nozawae (22 from Hokkaido Island and 2 from the Tohoku District) were sampled and examined Intrapopulational differentiation in the two species was measured by examining several indexes, including proportion of polymorphic loci (P), mean heterozygosity (H) and nucleotide diversity (π). All measurements were higher in theC. amblystomopsis populations, suggesting that intrapopulational variation inC. nozawae was less than inC. amblystomopsis and reflecting the difference in effective population sizes between them. Cluster analyses were performed using the UPGMA method, based on the data matrices of genetic distance (D) and the net nucleotide difference (δ) between populations. TheC. nozawae andC. amblystomopsis populations from Hokkaido Island composed a large cluster (Hokkaido group), while theC. nozawae populations from the Tohoku District composed a different cluster (Tohoku group). Bootstrap probabilities deduced from 1000 bootstrap replications for presence or absence of restriction sites showed that the mtDNA haplotypes detected within the Tohoku Group occurred in 99.9% of the bootstrap replicates outside the mtDNA haplotypes of the Hokkaido group, while those within the Hokkaido group occurred in 3.5–64.9% of bootstrap replicates. Consequently, the Hokkaido populations of the two species (Hokkaido group) were genetically close to each other, whileC. nozawae from the Tohoku District (Tohoku group) were distant from the Hokkaido group. These results suggest that the ancestral populations of the two species on Hokkaido Island shared the same gene pool, even after becoming geographically isolated from the ancestral population ofC. nozawae in the Tohoku District by the formation of the Tsugaru Straits.     

Morphological variation in hybrids between Salmo marmoratus, and alien Salmo species in the Volarja stream, Soca River basin, Slovenia

There were significant correlations between colour pattern, LDH -5*, genotype and certain meristic characters in 59 hybrid trout Salmo , sp. from the Volarja stream, Soca River basin, Slovenia. It is concluded that panmixia between native Salmo marmoratus , and introduced S. trutta , of Atlantic, Danubian and Mediterranean origin had not been reached in this zone, despite the long period of introgression. The result is in agreement with other studies dealing with introgression in Salmo , and for management purposes certain morphological characters, especially colour pattern, can be a valuable tool in restoring the marble trout population in the Soca River.     

Interspecific differentiation and intraspecific substructure in two closely related clupeids with extensive hybridization, Alosa alosa and Alosa fallax

In this study, the evolutionary relationships within and among populations of European shads, Alosa alosa and Alosa fallax , was investigated. Screening of allelic variation across eight allozyme loci and sequencing 448 bp of the mtDNA cytochrome b gene in 14 rivers throughout the range of the species supported that the two taxa were independent lineages (1·3% net nucleotide divergence) despite extensive hybridization. Genetic diversity and structure was considerably higher in A. fallax than A. alosa and the former species revealed evidence of distinct lineages in the Mediterranean and Atlantic basins. A Bayesian clustering approach combined with gill raker counts verified that individuals of the two species could be assigned to their parent group with relatively high confidence. Evaluation of hybridization in the Lima and Mondego Rivers in Portugal provided evidence that introgression is extensive but is not currently obscuring (through hybrid swarming) the diagnosability of the two species.     

Associations of allozyme variation and behavior in the cowpea weevil (Callosobruchus maculatus)

The cowpea weevil (Callosobruchus maculatus (Fabré)) exhibits several behavioral traits that are stable within, but vary among, strains. These traits are heritable and quantitative. We used cellulose acetate gel electrophoresis to quantify allozyme variation within and among laboratory cultures of four weevil strains and determine whether allozyme variation correlates with behavioral traits. Significant variation exists at 8 of 11 loci assayed and gene frequencies are significantly different among strains. The South Indian strain (SI) is most variable and measures of genetic distance set it apart from the other strains. It is also behaviorally unique. The Brazilian strain (BC) is most different from SI in allozyme diversity and behavioral phenotype, while two African strains (IITA, CAM) are intermediate in allozyme diversity and phenotype. These results are consistent with the known history of these strains and the differences in the allozymes parallel the differences in behavioral traits.     

Allozyme and microsatellite genetic variation in natural samples of zebrafish,Danio rerio

In this paper, we report the results from allozyme and microsatellite markers in natural populations of the zebrafish, a species of great significance in biological studies. Four zebrafish wild samples from West Bengal, India, were analysed in a preliminary survey of levels and patterns of genetic variation. Results indicate high levels of genetic variability and weak genetic structure, although the latter is consistent with the geographical features of the area under study, sampling sites being located in the Ganges and Brahmaputra delta region, which is characterized by high waterways connectivity.     

中国樱桃种内及与欧洲甜樱桃杂交的初步分析

以四川优良的中国樱桃栽培和野生种质及欧洲甜樱桃为试材进行杂交,对自交坐果率、杂交亲和性、杂交坐果率进行了调查和分析。结果表明:(1)中国樱桃自交亲和,欧洲甜樱桃自交不亲和;(2)杂交授粉2周后,各杂交组合表现出较高的杂交亲和性,坐果率为57.24~93.03%;5周后,各杂交组合表现出不同程度的杂种败育性,远缘杂交尤为明显;(3)在中国樱桃种内杂交中,以野生种质为父本的杂交坐果率极显著高于以栽培品种为父本的杂交坐果率;而远缘杂交中,以‘红灯’为母本,以中国樱桃栽培品种为父本的杂交坐果率显著高于以中国樱桃野生种质为父本的杂交坐果率;(4)受亲本影响,不同的杂交组合坐果率不同,受父本的影响极显著高于受母本的影响。基于上述结果,该研究进行了初步的分析和讨论。     

Experimental hybridization of Angiostrongylus mackerrasae, Bhaibulaya, 1968 and Angiostrongylus cantonensis (Chen, 1935)

Angiostrongylus mackerrasae and A. cantonensis are closely related and their status as distinct species was tested by experimental hybridization. Hybridization between A. mackerrasae and A. cantonensis was observed and the morphology of the F₁ hybrids was found to be intermediate between that of the parental species which suggests strongly that A. mackerrasae and A. cantonensis are both valid species. However, male F₁ hybrids were sterile, whereas females were fertile; no evidence of parthenogenesis or other form of asexual reproduction were observed in either species. It is suggested that A. mackerrasae and A. cantonensis may cross breed in nature.     

Numerical analysis of isozyme variation in Glycine tomentella

Isozyme variation among 114 accessions of the Glycine tomentella Hayata was analysed by single linkage cluster analysis and the unweighted pair group centroid method (UPGMC). The diploid accessions fell into six distinct, well defined groups, which conformed with differences in chromosome number (2n − 38 or 2n − 40) or in geographic origin. The majority of the tetraploid accessions belonged to a large, geographically widespread group, predominantly aneuploid (2n − 78) group. The remaining four tetraploid groups were distinct on the basis of morphology or geographic distribution. The validity of tetraploid isozyme groupings for reflecting subspecific differentiation was supported by the published reports of hybrid fertility. All of the nineteen crosses between isozyme groups have yielded sterile hybrids, whereas five crosses within groups have yielded fertile hybrids. The relationship between diploids and tetraploids was examined either as the similarity between individual accessions, or that between isozyme groups. These analyses indicated that each tetraploid group is closely related to only one or two of the diploid groups or subgroups.     

Allozyme variation in sympatric populations of British grasshoppers,—evidence of natural selection

Seven species of British grasshopper were analysed for variation at 15 enzyme loci. Where possible, sympatric populations were collected so that direct comparisons could be made. Allozyme frequencies in Chorthippus brunneus and C. albomarginatus were compared using correlation analysis. A positive correlation was noted for a phosphoglucomutase allozyme and a negative correlation for an isocitrate dehydrogenase allozyme. It was suggested that natural selection may affect allozyme frequencies in the wild.
Using cluster analysis and principal component analysis, evolutionary relationships of grasshoppers have been examined and compared with interpretations based on morphology.     

Rapid detection of natural cells of Alexandrium tamarense and A. catenella (Dinophyceae) by fluorescence in situ hybridization

The marine toxic dinoflagellates Alexandrium tamarense (Lebor) Balech and A. catenella (Whedon and Kofoid) Taylor that cause paralytic shellfish poisoning (PSP) are identified on the basis of morphological features in routine monitoring. Rapid and simple identification is, however, often difficult because of the morphological similarity. Fluorescent in situ hybridization (FISH) using ribosomal RNA (rRNA)-targeted probes has been studied as a method of easily identifying and enumerating species responsible for harmful algal blooms (HABs). Its application to monitoring natural populations of HAB species, however, is limited. Here, we applied the FISH method to identify and enumerate cells of A. tamarense and A. catenella in natural plankton assemblages collected from Japanese coastal waters. A. tamarense-specific (Atm1) and A. catenella-specific (Act1) probes were established based on the D2 region of the large-subunit ribosomal RNA gene (28S rDNA). With these two probes, natural cells of A. tamarense or A. catenella in field samples could easily be identified when the following three conditions were met. First, cells should be concentrated by filtration, not centrifugation, in order to avoid the loss of cells. Second, autofluorescence should be minimized; acetone was an effective decolorization reagent. Third, samples should be stored at −20 or −80 °C for long-term preservation. The results indicate that FISH is a useful tool for the rapid identification of toxic Alexandrium spp. and can facilitate the analysis of numerous natural samples.     

Chloroplast DNA variation,postglacial recolonization and hybridization in hazel,Corylus avellana

To unravel the postglacial migration history of hazel, Corylus avellana, the genetic variation at two types of chloroplast DNA markers, polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and microsatellites, was assessed in 26 natural hazel populations distributed across the range of C. avellana. In addition a sequence of 2468 base pairs, which contains the matK gene, was analysed in seven individuals. Very little variation was detected overall [hT:PCR-RFLP= 0.091, hT:microsatellite= 0.423, pi (nucleotide diversity) = 0.00093] but the microsatellite markers, which have the highest levels of variation, show a clear geographical structure that divides Europe into two areas: (i) Italy and the Balkans, on one hand and (ii) the rest of Europe, on the other hand. These data exclude Italy and the Balkans as possible origins of the postglacial recolonization but cannot unambiguously show which other area is the origin, since the genetic data does not indicate the direction of spread. If we take the pollen record into account, the most likely scenario would be an expansion from southwestern France into most of Europe except Italy and the Balkans, and then a local expansion in the latter area. The two main haplotypes identified with both PCR-RFLP and sequencing, A and B, were found not only in C. avellana but also in other European Corylus species and cultivars. Haplotype A, which is dominating all investigated natural populations of C. avellana, is also found in the European tree hazel (C. colurna) and haplotype B, which is rare in C. avellana, has been identified in the filbert (C. maxima) and C. avellana cultivars. This pattern seems to indicate a history of past hybridization among the European Corylus species and cultivars.     

Frequency and direction of hybridization in sympatric populations of Pinus taeda and P. echinata (Pinaceae)

Two naturally occurring, sympatric, northern Georgia populations of Pinus taeda L. (loblolly pine) and P. echinata Mill. (shortleaf pine) were examined with respect to genetic diversity within populations and the frequency, spatial distribution, and morphology of putative hybrids. Shortleaf pine predominated at the "road" site while loblolly pine predominated at the "granite outcrop" site. Hybrid individuals were identified by their IDH allozyme genotype, the only such locus known to be fixed for different alleles in the two species. All allozymatically detectable hybrids (34 at the road site and two at the granite outcrop site) were juveniles that were distributed in open, sunny patches. A similar pattern of recruitment was seen for juveniles of the parental species. Hybrids were spatially distant from mature shortleaf pine, suggesting that shortleaf pine was not the seed parent. Discriminant analysis on needle characteristics indicated that loblolly pine was easily distinguished from shortleaf pine and the hybrids, but that shortleaf pine and the hybrids were barely distinguishable from each other. A diagnostic cpDNA restriction site marker indicated that shortleaf pine sired all the hybrids at both sites. No evidence of later generation hybridization was found.     

云南秋海棠属植物有性杂交特性

通过云南秋海棠属植物属下5个系统分类组的种类及外来园艺品种共109个组合的有性杂交试验,以系统分类组合、园艺分类组合(不同茎的形态类型组合)分别进行方差分析。结果表明:云南秋海棠属植物属下等级的系统分类组内、组间的有性杂交,以及不同茎的形态类型的有性杂交均无显著差异,亲和力较强而可育性高,可在云南秋海棠属植物属内进行广泛地远缘杂交; 云南产秋海棠属植物原种与外来园艺品种间的有性杂交亲和性弱而可育性低,需选择花粉粒和胚囊正常发育的外来园艺品种大量杂交,并采用切实可行的技术措施克服远缘杂交不育,以期培育新颖奇特的秋海棠属植物新品种。     

Genetic variation and natural hybridization betweenOrchis laxiflora andOrchis palustris (Orchidaceae)

Genetic divergence between population samples ofOrchis laxiflora and ofO. palustris from various European locations was studied by electrophoretic analysis of 25 enzyme loci. An average genetic distance of D_Nei = 1.24 was found between the two taxa, with 12 out of 25 loci showing alternative alleles (diagnostic loci). Genetic heterogeneity was observed within bothO. laxiflora andO. palustris, when northern and southeastern populations were compared, being lower in the former taxon (D = 0.06), than in the latter (D = 0.16). Karyologically, 2n = 36 was found for bothO. laxiflora andO. palustris. O. laxiflora andO. palustris produce hybrids, described asO. ×intermedia. Genotype analysis of several sympatric samples showed the presence of hybrid zones, including F₁ hybrids and, in low proportions, recombinant classes, putatively assigned to F_n and backcrosses, as well as a few introgressed individuals of both taxa. These data indicate that hybrids are only partially fertile, with a very limited mixing up of the two parental gene pools; this is also shown by the lack of significant lowering of genetic distances when sympatric and allopatric heterospecific samples are compared. Accordingly,O. laxiflora andO. palustris form a syngameon; nevertheless they can be considered as good taxonomic species, with virtually distinct gene pools, which evolve independently. The genetic variability inO. laxiflora andO. palustris is remarkably low ( _e = 0.05 and _e = 0.02, respectively). In particular, nearly complete absence of polymorphic loci was found inO. palustris from northcentral Europe. Two hypotheses are considered to explain the low genetic variability of this endangered species.     

Molecular confirmation of natural hybridization between Lumnitzera racemosa and Lumnitzera littorea

The hypothesis of natural hybridization between Lumnitzera racemosa and Lumnitzera littorea, two mangrove species distributed in the Indo-West Pacific region, was proposed in 1970s based on morphological traits; however, no molecular evidence has been reported to support it. In this study, we sequenced two low-copy nuclear genes and one chloroplast intergenic spacer (trnS-trnG) in the two Lumnitzera species and their putative hybrid to test this hypothesis. Our results revealed that there were 9 and 27 nucleotide substitutions at the two nuclear loci, respectively, between one haplotype of L. racemosa and L. littorea, and that the putative hybrid showed additivity in chromatograms at these sites. Sequencing the chloroplast intergenic region trnS-trnG showed that the two Lumnitzera species differed by seven fixed nucleotide substitutions and four fixed insertions/deletions in this region, while the putative hybrid had identical sequences to L. racemosa. Molecular data clearly demonstrated that there indeed existed natural hybridization between L. racemosa and L. littorea and that L. racemosa was the maternal parent in this hybridization event. The uncommon direction of hybridization and F₁ nature of hybrids in this case, and in mangroves in general, is discussed.