Co-existence of Paragonimus harinasutai and Paragonimus bangkokensis metacercariae in fresh water crab hosts in central Viet Nam with special emphasis on their close phylogenetic relationship

During our epidemiological surveys for Paragonimus species in central Viet Nam, we found four morphologically different Paragonimus metacercariae in mountainous crabs. They were identified as metacercariae of Paragonimus westermani, P. bangkokensis, P. proliferus, and P. harinasutai in the order of their prevalence in crab hosts. This is the first discovery of P. harinasutai in Viet Nam, co-inhabiting with P. bangkokensis and other species. Metacercariae of P. harinasutai were given orally to a cat to obtain adult worms. Then, ITS2 and CO1 sequences of metacercariae and adults of P. harinasutai, and metacercariae of P. bangkokensis collected from the same place were determined for analyses of phylogenetic relationships to other P. harinasutai and P. bangkokensis populations as well as related species. The results of molecular analyses showed that P. harinasutai from Quang Binh province of central Viet Nam was almost completely identical with those from Vientiane, Lao PDR; P. bangkokensis from Quang Binh, Viet Nam was also almost completely identical with those from Lao PDR and from Quang Ninh province, Viet Nam. Except for one P. harinasutai isolate from China, all populations of P. harinasutai and P. bangkokensis from Thailand, Lao and Viet Nam make a single clade in both ITS2 and CO1 trees. In ITS2 sequences, AT deletion and ATC insertion were observed in some isolates of both species, indicating recent gene flow between P. harinasutai and P. bangkokensis. Moreover, because of their extremely high genetic similarities and their co-inhabitation in the same crab hosts found in Thailand, Lao PDR and Viet Nam, they should be considered as the sister species at the early stage of divergence. In addition, P. microrchis previously described from Yunnan, China should be placed as the synonym of P. harinasutai, because of their morphological and molecular similarities.     

Natural hybridization between black spruce and red spruce

Using species-specific random amplified polymorphic DNA (RAPD) markers and morphological characters, natural hybridization between the closely related black spruce Picea mariana (Mill.) B.S.P. and red spruce P. rubens Sarg. was evaluated in natural populations from north-eastern North America. Sampling included populations from both areas of allopatry and also 14 populations from part of the area of sympatry located in the province of Québec and covering several thousands of square kilometres. Classification results from RAPD species-specific markers and from a discriminant function based on morphology were compared. Molecular analysis of the allopatric populations indicated a small amount of interspecific gene leakage with no asymmetric directionality to introgression. A high occurrence of hybrid/introgressant individuals was observed within sympatric populations, suggesting weak reproductive isolation. As expected, the detection of such individuals was more efficient using molecular markers than with morphological traits. The hybrid zone appeared extensive with variable species structure and, in some stands, the main component composed of hybrid/introgressant trees. Implications for the genecology and genetic management of these species are discussed.     

Natural hybridization and conservation

This review deals with natural hybridization, an important subject in conservation biology. Natural hybridization is defined as the secondary contact between two populations that have evolved separately over a long period of time. This process is uncommon in terms of the total number of individuals involved, but is much less unusual if we consider the number of species that hybridize. Thus, natural hybridization may be an important process in the shaping of the evolutionary trajectories of many plant and animal species. The possible consequences of natural hybridization, which can either promote or prevent evolutionary divergence between taxa and will involve many ecological factors, are analysed here. I question whether natural hybridization poses always a problem in conservation and try to answer when conservation biologists and managers do have a responsibility to take decisions. Several examples of hybridization related to management strategies are also discussed. In conclusion, I believe that it is impossible to provide conservation managers with a simple handbook explaining how to proceed in cases of hybridization––each case is unique and should be analyzed individually. The only advice is that the more we know about hybridization and the factors involved, the better we will be able to assess each situation, to establish the possible consequences and even to estimate the probability of success of any particular conservation strategy.     

Natural hybridization in JapaneseCalamagrostis

Morphological and pollen studies were made for the collections belonging toCalamagrostis langsdorffii, C. Sachalinensis, C. longiseta, C. nana and various intermediates between these species from central Honshu, the chromosome numbers of which were reported in a previous paper (Tateoka, 1976). Additional collections made from the Akaishi Range in 1976 were also subjected to the present work. The intermediates showed 0%, or nearly 0%, pollen stainability and a mosaic of morphological features of putative parental taxa which can be estimated to occur for F₁ hybrids. This estimation was sustained by a comparison with the pattern of character expressions for the artificially raised F₁ hybrids of various European species ofCalamagrostis as reported by Nygren (1962). These results as well as field observations strongly suggested that the intermediates may be wholly or almost wholly F₁ hybrids. The hybrids were sometimes found in abundance within a limited area, but seemed to have little capacity to migrate from the places where they originated. The following combinations, which were not previously recorded, were disclosed:C. longiseta×C. sachalinensis, C. langsdorffii×C. nana, C. langsdorffii×C. longiseta, C. longiseta×C. nana. Possibilities of introgression in these hybridizing species were discussed. The hybrids were tetraploids except one hexaploid which was discovered in a mixed population ofC. nana, C. longiseta and their putative F₁ hybrids at the tetraploid level. The hexaploid in question, which seemed to have resulted from the participation of an unreduced gamete, was very similar morphologically toC. nana subsp.hayachinensis distributed in the mountain far distant from central Honshu. Geographical distributions of the hybrid plants were surveyed through examination of the herbarium specimens.     

Natural hybridization in JapaneseCalamagrostis

Taxonomic relationships betweenCalamagrostis langsdorffii andC. sachalinensis were investigated in connection with the discovery that a considerable number of plants intermediate between these species occur in the high mountains of central Honshu. The studies were mainly carried out on the basis of cytological voucher specimens for which a chromosome number was known. Their morphological features, leaf flavonoids and pollen grains were subjected to examination. Hybridization, polyploidy and gametophytic apomixis are related to the organization of this complex, and plants referable to the “intermediates” are apomictic octoploids superimposed on the ancestral and sexual tetraploid taxa. The internal structure of this complex is discussed together with its probable course of evolution, and a diagram showing the outline of the structure is presented. The variability of morphological features and flavonoid patterns of the plants concerned is examined, and the delimitation ofC. langsdorffii andC. sachalinensis is made clearer.     

Natural hybridization in the coccinellid genus chilocorus

A population of lady-bird beetles at the head of the Crowsnest River valley on the immediate east side of the Rocky Mountains has been sampled regularly over the years 1961 to 1964. The 389 individuals studied had chromosome numbers ranging from 2n = 14 (58%) to 2n = 20 (3%). The former is the diploid number of C. hexacyclus, the latter that of C. tricyclus, species otherwise known respectively only in the Prairie Provinces and the interior of British Columbia plus the adjoining part of the State of Washington. About 19% of the beetles had the intermediate number of 17 chromosomes, males possessing either a hexacyclus-type or a morphologically distinct tricyclus-type Y chromosome. — In all numerical categories meiotic pairing was virtually perfect but the trivalents in heterozygotes were frequently linearly aligned. This assures production of aneuploid (duplicationdeficiency) gametes, only complementary recombinations of which are viable. Segregational sterility resulting from triple (92%), double (84%) and single (50%) heterozygotes is estimated to be ca 40%. Of four neo-diploids, with zero chromosomal sterility, predicted in the F₂ only one materialized with certainty, the one with 13 + tricyclus-type Y. This is interpreted to mean that F₂ progeny are rare; rare because (1) random mating, complete introgression, is the rule and (2) Segregational sterility is severest in triple heterozygotes. — Despite the population-depressing effect of the inviable aneuploid zygotes, it is calculated that segregation and crossing-over generate a minimum of 148 × 10⁹ new recombinants (a minimum were crossing-over strictly localized); those acquiring beneficial attributes will be favoured by natural selection. — Topographically, it is evident that tricyclus adults invade the resident hexacyclus community year after year, borne on the prevailing westerly winds that funnel through Crowsnest Pass (elevation only 4463): hexacyclus is thereby constrained to the east of the Continental Divide and consequently does not penetrate into tricyclus territory.Dedicated to Prof. J. Seiler on the occasion of his eightieth birthday.     

Natural hybridization and taxonomy of birches in North Asia

Currently there is a complicated situation in the taxonomy of widespread species such as birch (Betula L.). A large number of species of this genus in Siberia and the Russian Far East has been described, but one encounters an insurmountable obstacle when trying to carry out a more detailed study: the impossibility of finding them for a second time in nature or at least collect plants that have identical characteristics to the typical examples of the described species. All of these species are characterized by a very high variability of traits. These are the traits of Betula pendula and B. microphylla in the southern parts of Central Siberia and Betula pendula (= B. platyphylla) and B. lanata in Eastern Siberia and in the northern regions of the Russian Far East. This suggests intense and widespread hybridization between these species. In connection with this, research of this genus throughout North Asia has begun. These studies have been carried out over 40 years. To date, many works in periodicals and several monographs have been published using the data of numerous expeditions. This paper shows the role of natural hybridization in plant taxonomy using the example of the Siberian species of birch. It is necessary to have a clear understanding of the hybridization processes on large areas that include the ranges of contact species. Solving problems of taxonomy only by studying small local populations is a hopeless pursuit.     

Natural hybridization between species of Calceolaria with different pollination syndromes in southern Patagonia, Argentina

 Calceolaria uniflora and C. polyrhiza differ markedly in flower structure, color, and in their pollination mechanisms. Flowers of C. polyrhiza have a hidden glandular oil-secreting appendage that in the genus typically attracts bee pollinators, while in C. uniflora this is replaced by an external, white and fleshy structure that functions as food body and attracts a bird pollinator. Populations with intermediate flower morphology, presumably hybrids, were found in several sites over a large area of southern Patagonia, Argentina. The intermediate forms have so far been found always in contact or close proximity with typical populations of both presumed parent species. The distribution pattern and phenetic intermediacy in several independent flower characters strongly suggest widespread interspecific hybridization in the range of geographic overlap of the two species. The pattern of variation of intermediate forms in some sites is indicative of a hybrid swarm and possible introgression. Of particular interest is the range of intermediate structures and positions of the flower appendage, between oil gland and food tissue, that can be found in hybrid populations. The observed phenomenon raises several questions concerning the biological mechanisms generating it and its evolutionary consequences. Received May 31, 2001 Accepted September 4, 2001     

Natural hybridization and gene flow between twotridentiger gobies in lake hinuma (ibaraki prefecture, japan)

Of 851 specimens ofTridentiger obscurus andt. brevispinis collected from Lake Hinuma (Ibaraki Prefecture, Japan) from July 1996 to February 1998, 49 (5.8%) comprised F₁ hybrids and backcross progeny of the two species. Since the mitochondrial DNA haplotypes of the F₁ hybrids reflected those ofT. brevispinis, most instances of hybridization are thought to have occurred between maleT. obscurus and femaleT. brevispinis. Although both allozyme and mitochondrial DNA analyses indicated backcrossing and introgression of mitochondrial DNA, the frequency of backcross progeny was relatively low, suggesting the existence of a natural selection to backrossing.     

Evidence for metacercarial polymorphism in lung flukes, Paragonimus ohirai and Paragonimus iloktsuenensis

Cross breeding experiments between 2 species of lung fluke, Paragonimus ohirai and Paragonimus iloktsuenensis, were carried out using metacercarial characteristics as distinguishing markers. All metacercariae of F1 obtained were identical to those of P. ohirai. In the F2 and BF1 of F1 X P. iloktsuenensis, both the P. ohirai and P. iloktsuenensis types of metacercariae appeared. In the BF1 of F1 X P. ohirai, however, only the P. ohirai type metacercariae were produced. No intermediate type between the 2 species appeared. The results obtained demonstrate that the differences in the metacercariae, which were previously regarded as the most important characteristic for specific discrimination between these 2 flukes, are only a hereditary phenomenon within a single species. The metacercarial form, number of cyst layers, and body size seem to be controlled by a couple of alleles or very closely linked genes following simple Mendelian inheritance. Furthermore, we confirmed that reproduction in the lung flukes depends on cross-fertilization.     

Natural hybridization betweenSaimiri taxa in the Peruvian Amazonia

A wild population situated in the border of the distributions ofSaimiri sciureus macrodon andS. boliviensis peruviensis, in the Peruvian Amazonia, was studied in relation to 22 protein loci. These genetic markers provided indications of secondary intergradation between these two taxa, reinforcing previous morphological and cytogenetic evidences. Continued studies in this region on the hybrids' viability and fertility may be important for decisions related to the taxonomy of this genus.     

Intergeneric hybridization between Psathyrostachys and Hordeum

A total of 38 intergeneric combinations with Hordeum and Psathyrostachys was attempted. Generally the seed set was high, and 23 new intergeneric hybrid combinations were synthesized by the aid of embryo culture technique. All combinations showed complete sterility. The meiotic pairing behaviour in 8 combinations is reported. On the basis of these data it is clear that the N genome in Psathyrostachys is distinct and shows no genomic relationship, neither with the "X" and I nor with the H genomes in Hordeum.     

Spontaneous hybridization between maize and teosinte

The closest wild relatives of maize, Zea mays ssp. mays are various Zea taxa known as "teosinte." Hybrids between maize and the teosinte taxon, Zea mays ssp. mexicana, often occur when the 2 are sympatric in Mexico. Measuring the spontaneous hybridization rate of the 2 taxa would shed light on the mechanisms contributing to the evolution and persistence of these hybrid swarms. We conducted a series of field experiments in Riverside, CA, to measure the natural hybridization rates between maize and 2 teosinte taxa, Z. m. ssp. mexicana and Zea mays ssp. parviglumis. We planted teosinte within and near maize plantations. Hybrids were identified by progeny testing for a maize-specific herbicide resistance allele and a teosinte-specific allozyme allele. Hybridity was confirmed by growing putative hybrid progeny to maturity to evaluate whether they had the characteristic morphology of maize x teosinte hybrids. We found that maize and Z. m. ssp. mexicana naturally hybridize at a low rate (<1%), whereas Z. m. ssp. parviglumis hybridizes with the crop at a high rate (>50%).     

Natural hybridization between 2 sympatric species of mice, Mus musculus domesticus L. and Mus spretus Lataste

Using protein loci and DNA markers, we show by a multilocus genetic analysis that certain populations of the two sympatric mouse species Mus musculus domesticus and Mus spretus show clear signs of partial introgression. Given the sterility of F1 males and the known partial genetic incompatibilities between the genomes of the two species, our finding does not invalidate the biological species complex, but allows to think that very limited genetic exchanges remain possible even long after the divergence of taxa. This may have some consequences on the dynamics of certain kinds of invasive or advantageous DNAs like transposable elements or pathogen resistance genes.     

Natural hybridization in the genus Colletia (Rhamnaceae)

Natural hybridization in the genus Colletia (Rhamnaceae). Natural hybridization between two Chilean species of Colletia is assumed on the basis of morphological evidence. The characters of both species, C. hystrix Clos and C. ulicina Gill. & Hook., and those of their putative hybrids are described. The morphological intermediacy of the assumed hybrids is tested through Wells' distance diagram.     

Natural hybridization between an outcrossing and a selfingPhlox (Polemoniaceae): The maternal species of F1 hybrids

Natural F₁ hybrids between the outcrossingPhlox drummondii and the predominantly selfingP. cuspidata were examined to ascertain the proportion of hybrid individuals mothered by each species. Species-specific restriction fragment patterns (both nrDNA and cpDNA) were established as markers, and synthetic hybrids of known parentage were utilized to determine that the chloroplast genome is maternally inherited. Of 89 mature natural hybrids examined, approximately two thirds were mothered byP. drummondii, the outcrosser. That the outcrosser should mother most hybrids is expected since it is dependent upon incoming pollen for fertilization, and hybrids may result when heterospecific pollen is received. The fact that the highly selfingP. cuspidata mothered nearly one third of the hybrids is surprising, and may be related to both pre-zygotic and post-zygotic factors. Which species mothers hybrids has important implications for the potential for introgression as well as its direction.