Interspecific Hybrid Sterility Research Articles

Observations of Botrychium Xwatertonense and Its Putative Parent Species, B. hesperium and B. paradoxum

We observed mapped individuals of three species of Botrychium Sw. at a single site in Waterton Lakes National Park, Alberta for six years. Our study showed that the interspecific sterile hybrid, B. xwatertonense W.H. Wagner and its putative parents, B. hesperium Maxon & R.T. Clauson and B. paradoxum W.H. Wagner, are stable, unique genotypic entities. For B. hes- perium and B. Xwatertonense, the degree of sporophore pinnation and the number of trophophore pinnae are plastic characters with much of the observed variation explained by the size of the plants. Although B. Xwatertonense is reported to be sterile, most sporophytes appeared more than 4 cm from other conspecifics, indicating that they are probably not produced vegetatively. Mean height of B. Xwatertonense was greater than that of either of its parents, indicating hybrid vigor. Many members of the Botrychium subg. Botrychium are considered rare (Lellinger, 1985). Nine species in the continental United States are can- didates for listing as threatened or endangered species under the Federal En- dangered Species Act of 1973 (USDI-Fish and Wildlife Service, 1993), and nine species are listed as rare in Canada (Argus and Pryer, 1990). Often rare species occur together with more common species in genus (Wagner and Wagner, 1983). However, these communities are natural common garden experiments and can be a tool to evaluate the discreteness of conge- neric species growing together. Species that maintain their morphological dif- ferences in communities are evidence that these differences are genet- ically fixed. The sterility of interspecific hybrids is additional evidence of ge- netic differentiation between the parent species (Wagner and Wagner, 1983). Members of Botrychium are often difficult to differentiate because the plants are simple and offer few characters for recognizing species (Cody and Britton, 1989). Furthermore, intraspecific variation in leaf shape within and among populations and among years is not well understood (Graham and Wagner, 1991). Keys to the taxa of Botrychium subg. Botrychium include sporophore and trophophore characters (Wagner and Wagner, 1986), but the range of vari- ation among species for these characters is often not known. The number of hybrids found in communities is usually very small (Wagner, 1991). One reason for the paucity of hybrids may be the low fre- quency of intergametophytic fertilizations (Soltis and Soltis, 1986). However, at the B. Xwatertonense type locality studied by Wagner et al. (1984), almost 25% of the plants were sterile hybrids. The reason for this high frequency of

A theoretical assessment of recombinational speciation

Using a computer simulation, we have examined the dynamics of recombinational speciation, a potentially rapid mode of evolution dependent on chromosomal reassortment in populations of partially sterile interspecific hybrids. We describe how various parameters affect the time required for a new recombinant species to become established within the setting of a spatially structured hybrid zone. Our results indicate that recombinational speciation is most likely to occur where (1) the hybrid zone interface is long, (2) the organisms involved are predominantly selfing, (3) the hybrids are relatively fertile, and (4) the number of differences in chromosomal structure between the parental species is small. The speciation dynamics are characterized by long-term stasis followed by an abrupt transition to a new reproductively isolated type. The results are largely the same whether the nascent recombinant species is favoured by a fertility or a viability advantage. Recombinational speciation, like polyploidy, appears to be a feasible mechanism for sympatric speciation in plants.

Lutzomyia longipalpis is a species complex: genetic divergence and interspecific hybrid sterility among three populations.

The sand fly Lutzomyia longipalpis is the vector of Leishmania donovani chagasi in Latin America. An analysis of genetic variability at 27 enzyme coding loci among three laboratory populations of Lu. longipalpis revealed substantial genetic polymorphism. Levels of genetic distance between all pairwise comparisons of colonies were very high, and consistent with those previously reported among separate species in the genus Lutzomyia. Between 7% and 22% of the loci studied were diagnostic for any two of the colony populations. Experimental hybridization between colonies resulted in the production of sexually sterile male progeny. Our results provide strong evidence that Lu. longipalpis exists in nature as a complex of at least three distinct species. The possible effects of colonization on the genetic makeup of laboratory populations is considered in extending our results to natural populations.

A Simple Method to Test Genetic Allelism in Nearly Sterile Interspecific Plant Hybrids

The comparison of electrophoretic isozyme patterns of multimeric enzymes in leaf and pollen extracts from the same individual has been used to distinguish allozymes and isozymes. This is possible because in genetic heterozygotes leaf tissue exhibits both homomers and heteromers whereas pollen shows only homomers. I show that pollen of nearly sterile interspecific plant hybrids contains isozymes that can be examined in the same way. This permits a test to determine whether genes encoding particular isozymes in the parental species have remained allelic; i.e., located at corresponding positions on homologous chromosomes. Such a test is illustrated by examining cytosolic PGI in two classic hybrid combinations in Clarkia. The evidence shows that the coding PGI genes of the parental species are located on different chromosomes (or at different sites on the same chromosome), possibly resulting from reciprocal translocation as proposed by Lewis and Raven. The ability to determine whether genes in different species segregate as alleles or not will likely be useful in studying the extent of gene linkage conservation and as a supplement to RFLP and other DNA-directed maps. Formal genetic analysis is generally restricted to crosses between individuals of a single species because fertile hybrids are necessary to produce an F2 or other segregating progeny. However, for genes encoding multimeric enzymes expressed in pollen and detectable by electrophoretic assays, I show here that tests of allelism can be carried out between species even when their hybrids have very low fertility. This new test is an extention of previous results in which the electrophoretic isozyme pattern of extracts from leaves or other somatic tissues is compared with that of pollen from the same individual (Weeden and Gottlieb 1979). The test depends on the fact that following meiosis the haploid cells that develop into pollen receive one or the other allele at every locus, but not both. Thus if a particular locus is heterozygous for alleles encoding allozymes having different electrophoretic mobilities, leaf tissue exhibits both homomers and heteromers, but only homomers are synthesized in the pollen because any single grain receives only a single allele. For a dimeric enzyme in heterozygous individuals, extracts from a mass of pollen exhibit two homodimers but not the electrophoretically intermediate heterodimer. Its absence is apparent when patterns from the pollen and leaves are compared. In contrast, if the two homodimers are encoded by two different homozygous loci, then both leaf tissue and pollen show the same three-banded pattern. The pollen-leaf comparison has proved useful to test allelism of electrophoretic variants of numerous enzymes within plant species in which formal breeding studies cannot be readily carried out (Crawford 1990). The allelism of variants in different species can also be tested by the pollen-leaf comparison because inviable pollen often characteristic of interspecific hybrids contains many functional enzymes detectable by electrophoresis. In this report, I demonstrate the use of the pollen-leaf test to assess allelism of electrophoretic variants in two classical interspecific hybrids in Clarkia: C. franciscana x amoena and C. amoena x rubicunda (Lewis and Raven 1958a, 1958b). MATERIALS AND METHODS Plants. Interspecific hybrids were produced by crossing Clarkia amoena subsp. huntiana (Jepson) Lewis & Lewis x C. rubicunda (Lindley) Lewis & Lewis and x C. franciscana Lewis & Raven. Clarkia amoena subsp. huntiana was collected in Lake Co., near Manning Creek Bridge on Rt. 175, west of the junction with Rt. 29 (LDG 9025), and in Marin Co., on Lucas Valley Rd., 5.8 miles east of Nicasio Schoolhouse (LDG 714). Clarkia rubicunda was collected in Marin Co., at Ft. Barry (LDG BAR). Clarkia franciscana was represented by two populations, the type locality at The Presidio in San Francisco Co., and a newly discovered population in the foothills above

A test of reciprocal X-Y interactions as a cause of hybrid sterility in Drosophila.

Elucidation of the nature of the gene interactions that underly the sterility of interspecific hybrids is important in evolutionary biology. The interactions between the heterospecific X and Y (or Z and W) chromosomes are often used as an explanation for two reasons. First, the fertility of the hybrids of the heterogametic sex is much more often affected than that of the homogametic sex (Haldane's rule) and X-Y interactions are specific to the heterogametic sex. Second, sex chromosomes, especially the X chromosome, are often considered to be of special importance in determining the fertility of hybrids. X-Y interactions have been addressed in studies of males with a heterospecific Y chromosome in a mixed genetic background. A more stringent test of the X-Y interaction model requires each X chromosome sterility factor to be tested separately for its interaction with the Y chromosome in a homogeneous background of the pure species. Here we report such a test of the X-Y interaction model and conclude that X-Y interactions should not be assumed to be the only or even the most common cause of hybrid sterility.

Deficiency of X and Y chromosomal pairing at meiotic prophase in spermatocytes of sterile interspecific hybrids between laboratory mice (Mus domesticus) and Mus spretus.

The normal association between the X and Y chromosomes at metaphase I of meiosis, as seen in air-dried light microscope preparations of mouse spermatocytes, is frequently lacking in the spermatocytes of the sterile interspecific hybrid between the laboratory mouse strains C57BL/6 and Mus spretus. The purpose of this work is to determine whether the separate X and Y chromosomes in the hybrid are asynaptic, caused by failure to pair, or desynaptic, caused by precocious dissociation. Unpaired X-Y chromosomes were observed in air-dried preparations at diakinesis, just prior to metaphase I. Furthermore, immunocytology and electron microscopy studies of surface-spread pachytene spermatocytes indicate that the X and Y chromosomes frequently fail to initiate synapsis as judged by the failure to form a synaptonemal complex between the pairing regions of the X and Y chromosomes. Several additional chromosomal abnormalities were observed in the hybrid. These include fold-backs of the unpaired X or Y cores, associations between the autosome and sex chromosome cores, and autosomal univalents. The occurrence of abnormal autosomal and XY-autosomal associations was also correlated with cell degeneration during meiotic prophase. The primary breakdown in hybrid spermatogenesis occurs at metaphase I (MI), with the appearance of degenerated cells at late MI. In those cells, the X and Y are decondensed rather than condensed as they are in normal mouse MI spermatocytes. These results, in combination with the previous genetic analysis of spermatogenesis in hybrids and backcrosses with fertile female hybrids, suggest that the spermatogenic breakdown in the interspecific hybrid is primarily correlated with the failure of XY pairing at meiotic prophase, asynapsis, followed by the degeneration of spermatocytes at metaphase I. Secondarily, the failure of XY pairing can be accompanied by failure of autosomal pairing, which appears to involve an abnormal sex vesicle and degeneration at pachytene or diplotene.

Production and characterization of Solanum amphidiploids and their resistance to bacterial wilt

Efforts to generate amphidiploid hybrids between eggplant ( Solanum melongena L. and Solanum integrifolium Poir. were made in order to obtain easily propagated and disease-resistant rootstocks for eggplant and tomato production. In vitro treatment of shoot tip explants of sterile interspecific hybrids, between S. melongena and S. integrifolium, with 0.05% colchicine induced a high percentage of amphidiploids. The amphidiploids had larger anthers containing approximately 70% viable and larger pollen than those of the interspecific hybrids. The amphidiploid plants produced a profuse number of viable seeded fruits, while the interspecific hybrids produced no fruit or a poor number of parthenocarpic fruits. The seeds of the amphidiploids were bigger than those of the interspecific hybrids. Chromosome number from pollen mother cell and root tips, nucleus volume of the histogenic layers and stomata size assisted in identifying true amphidiploids and chromosomal chimeric types. Seed-grown amphidiploid seedlings were found resistant to two of the most virulent strains of bacteria.

Sterility of male and female hybrids of Drosophila virilis and Drosophila lummei

The sterility of interspecific hybrids between the sibling species Drosophila virilis and D. lummei was tested in reciprocal F1s and different second generation combinations of cytoplasm, sex chromosomes and autosomes. Males with motile sperm and females with at least one mature egg were scored as fertile. When D. virilis was the mother, about 5 per cent of the F1 male progeny was sterile, the reason being that the X of D. virilis was weakly incompatible with the heterozygous second, fourth and fifth chromosomes. The F1 males carrying the X chromosome of D. lummei were all fertile. The X chromosome of D. lummei, and in particular its species-specific double inversion In(1)a + b, nevertheless caused sterility in 70 per cent of males in the presence of homozygous autosomes of D. virilis. Sterility was rare among the females, although some weakly disadvantageous X versus autosome and autosome versus autosome interactions were detected. The male and female hybrid sterilities were based on different genetic systems. The results emphasize the central role of the X chromosome in hybrid male sterility.

In vitro multiplication of a sterile interspecific hybrid, Brassica fruticulosa x B. campestris

In vitro methods for plant multiplication of a sterile interspecific hybrid between Brassica fruticulosa and B. campestris through either micropropagation or callus regeneration is described. Shoot-tip, single-node and leaf explants, obtained from in vitro-grown hybrids, regenerated on media containing NAA and BA. In vitro application of colchicine induced chromosome doubling in in vitro-regenerated shoots resulting in the production of fertile amphidiploids. Comparative studies on regeneration potential of the hybrid and its parents were also carried out using callus from leaf explants. The explants of B. fruticulosa and the hybrid were capable of shoot and root formation while those of B. campestris failed to form shoots but produced profuse roots. The results demonstrate the efficacy of an in vitro method in producing a large number of hybrid plants and fertile amphidiploids from incompatible crosses that yield very few hybrid seeds/seedlings.

Zinnia Marylandica (Asteraceae: Heliantheae), a New Disease-Resistant Ornamental Hybrid

Zinnia marylandica, an artificial hybrid betweenZ. angustifolia var.angustifolia (2n=22 female) andZ. violacea (2n=24, male), is described and illustrated.Zinnia marylandica is a stabilized amphiploid (2n=46) produced by colchicine-induced doubling of the sterile interspecific hybrids. It exhibits disease resistance to powdery mildew (Erysiphe cichoracearum), alternaria blight (Alternaria zinniae), and bacterial leaf and flower spot (Xanthomonas campestris pv.zinniae).

Electrophoretic Evidence for Interspecific Hybridization in Polystichum

Hybridization and polyploidy in Polystichum (Dryopteridaceae) have contributed to the morphological diversity and taxonomic complexity in this widespread genus. Patterns of reticulate evolution are particularly evident in the Polystichum complex from western North America (W. Wagner, 1973; D. Wagner, 1979), and several sterile interspecific hybrids have been reported (see P. Soltis et al., 1987, for review). As part of a comprehensive molecular analysis of polyploidy, its causes, and genetic consequences, we have studied two additional interspecific hybrids. In this paper we provide electrophoretic documentation of hybridization between P. andersonii and P. munitum and between P. lemmonii and P. munitum. Both hybrids were previously reported based on morphological and cytological evidence (W. Wagner, 1973). Polystichum andersonii occupies lowland coastal forests in British Columbia and southeastern Alaska, montane forests in western Washington and Oregon, and also occurs disjunctly in southeastern British Columbia, northern Idaho, and northwestern Montana. This species is tetraploid (2n= 164; W. Wagner, 1973; D. Wagner, 1979), but its ancestry is uncertain. Warren Wagner (1973) suggested that this species is of autopolyploid origin based on its distinctive leaf morphology and the presence of a vegetative bud on the rachis of the frond. However, cytological data (W. Wagner, 1973) show that P. andersonii contains a chromosome complement homologous with the common diploid P. munitum, suggesting instead an allopolyploid origin for P. andersonii. Anatomical, chromatographic, and electrophoretic data also support this hypothesis (D. Wagner, 1979; P. Soltis et al., unpubl. data). Although P. munitum is a likely candidate for one of the diploid progenitors of P. andersonii, the identity of the other parental species is currently unknown, although D. Wagner (1979) described a specimen from northern British Columbia that seems to fit the predicted morphology of the second progenitor. Polystichum lemmonii (2n = 82) occurs on open, serpentine, montane slopes in northern California, southwestern and central Oregon, and central Washington. Following D. Wagner (1979), P. lemmonii is herein considered distinct from the South American P. mohrioides.

Cytogenetic studies of the F1 hybrids of Capsicum annuum with C. chinense and C. baccatum

Partially sterile interspecific hybrids were obtained between C. annuum var. 'cerasiformis' and C. chinense var. 'mishme' (H1), and C. annuum var. 'cerasiformis' and C. baccatum var. 'pendulum' (H2). Morphologically the F1 hybrids were intermediate between the corresponding parents. Meiosis was irregular in the two F1 hybrids. Cytological analysis of the two F1 hybrids revealed that the genome of C. annuum differs from C. chinense by two translocations and some minor structural alterations and from C. baccatum by two translocations, a single inversion and some minor structural alterations. Isolation barriers such as hybrid inviability, weakness and hybrid breakdown in the H1 hybrid and, inaddition, desynapsis in the H2, were operative in these taxa. The differences between the present findings and those reported earlier on the two F1hybrids were attributed to differences in the genetic architecture of the taxa employed in hybridization.

The genomic relationships among six wild perennial species of the genus Glycine subgenus Glycine Willd.

Based on meiotic chromosome behavior in intra- and interspecific hybrids, genome symbols were assigned to the following diploid (2n=40) Glycine species: G. canescens = AA; G. clandestina- Intermediate pod (Ip)=A 1 A 1; G. clandestina-Short pod (Sp)=BB; G. latifolia = B 1 B 1; G. tabacina = B 2 B 2; G. cyrtoloba = CC; and G. tomentella = DD. Genome symbol GG was reserved for the soybean, G. max. At metaphaseI, loose chromosome associations were observed in completely sterile interspecific hybrids whose parents differed in their genomes, suggesting some chromosome homologies among species. Although G. clandestina-Sp, G. latifolia and G. tabacina are morphologically distinct species, they differ only by a paracentric inversion. Similar observations were recorded for G. canescens and G. clandestina-Ip. Evidence is presented that demonstrates that G. tabacina (2n=80) and G. tomentella (2n=78, 80) are allotetraploid species complexes. Hybrid weakness, sterility, seedling lethality and seed inviability were found in intra- and interspecific hybrids.

The inheritance of resistance to powdery mildew in interspecific hybrids and induced amphiploids of Zinnia elegans Jacq. and Z. angustifolia HBK

Sterile interspecific hybrids and colchicine-induced amphiploids of Zinnia elegansJacq. and Z. angustifolia HBK were examined to determine the mode of inheritance of resistance to Erysiphe cichoracearum DC ex Merat. Fertility was restored through colchicine treatment of two sterile hybrids of species reciprocal parentage which differed in ray petal response to the pathogen. Derived amphiploids were subsequently intercrossed to overcome the lack of segregation for this trait due to genetic control of pairing upon chromosome doubling. Resistance to E. cichoracearum appears to be complexly inherited in both leaves and ray florets of sterile hybrids and induced amphiploids. Two major dominant genes have been implicated in conferring resistance in ray petal tissue of derived amphiploids. Data obtained from the F1 hybrid progeny of the intercrossed amphiploids indicate that this trait is not cytoplasmically inherited. It is speculated that the genes conferring resistance in the ray florets are acting independently from those controlling leaf resistance and that most, if not all, of the resistance genes are inherited from Z. angustifolia.

ユリの種間雑種の雑種不稔性の克服

For the purpose of overcoming interspecific hybrid sterility in Lilium, the following methods were attempted: 1) Further use of embryo culture in the cross of F1 and 2) Use of amphidiploids induced from F1. When F1 hybrids were used as female parents, embryos were developed in 0-3.4% of their ovules. A total of 134 seedlings from nine backcross (or the similar cross) combinations were obtained through embryo culture. A seedling from the cross of F1 as male parent flowered and proved to be a triploid. Amphidiploids could be used as pollen donors and eight B1 plants were produced from the cross of the amphidiploid of L. longiflorum×L. henryi.Three new interspecific hybrids in Lilium produced by embryo culture were introduced.

Systematics of Solanum sect. Solanum (Solanaceae) in North America

Numerical taxonomic studies show that eleven species of Solanum sect. Solanum occur in North America. Five diploid (n = 12) species, S. americanum, S. douglasii, S. interius, S. pseudogracile, and S. ptycanthum are apparently native. A sixth diploid species, S. sarrachoides, is an introduction from South America. Three polyploid species, S. furcatum, S. nigrum, and S. villosum, occur infrequently. Two other polyploid species, S. scabrum and S. retrofiexum, are sometimes cultivated. A notable nomenclatural change, necessitated by a previously selected lectotype for S. americanum, is the use of the name S. americanum for the species previously known as S. nodfiorum. The name S. ptycanthum is now used for the species of the north- eastern United States formerly called S. americanum. Solanum sect. Solanum (also known as the S. nigrum complex) is a cos- mopolitan group of about 30 annual or short-lived perennial, herbaceous species of weeds. A variety of historical and biological factors combine to make the taxonomy of this section difficult (Edmonds 1979a). The use of experimental taxonomic techniques in several regional studies has clarified the taxonomy of this section in much of its range (Edmonds 1972, 1979a; Henderson 1974; Heiser et al. 1979). The present study covers the North American species of the section. Sterility of interspecific hybrids and ease of obtaining hybrids in sect. Solanum make it tempting to attempt to define species on the basis of genetic isolating mechanisms. However, the high frequency of intraspe- cific hybrid sterility and its lack of correlation with morphological diver- gence indicate that hybrid sterility is not useful in delimiting species in this section (Schilling and Heiser 1979). Because the breeding system of most species of sect. Solanum is primarily autogamous (Schilling 1978), there is probably little gene flow between populations with the result that biological species (Mayr 1963) are not present in this section. Hence, although data from crossing studies have been incorporated into this study as phenetic characters, they are not accorded any special signifi- cance as evidence of an isolating mechanism that separates species. To classify species in this section, an operational approach using techniques of numerical taxonomy to discover discontinuities by which species may be recognized has been used.

Production of tetraploid hybrids betweenOrnithopus pinnatusandO. sativususing embryo culture

Abstract Four sterile diploid interspecific hybrids were obtained by pollinating precociously exserted stigmas of Ornithopus pinnatus with pollen from O. sativus while buds were still closed, and culturing the resulting hybrid embryos in vitro with nurse endosperm. Doubling the chromosome number of one hybrid by treatment of shoot tips with colchicine gave partial restoration offertility. In vitro embryo culture was necessary to raise naturally selfed F2, F1, and F4 generations as endosperm development was impaired. However, occasional F4 seeds showed almost normal endosperm development, suggesting that in situ maturation of seeds may be achieved in future generations. The project was aimed at combining the high productivity of O. sativus with the prostrate habit, hard-seededness, and foliar tannin production of O. pinnatus. F1 and F4 individuals derived from a single tetraploid F2 plant showed a combination of the leafiness of O. sativus with a trailing habit more like that of O. pinnatus and variation i...

The genomes of Arachis hypogaea. 1. Cytogenetic studies of putative genome donors

Cytological studies of wild diploid Arachis species in the same section of the genus (sect. Arachis) as the cultivated peanut A. hypogaea L. show, with one exception, a karyotype characterized by the presence of 9 pairs of larger chromosomes and one pair of small (‘A’) chromosomes. The exceptional species A. batozocoiKrap. et Greg. has a more uniform karyotype. Interspecific hybrids between diploid species of similar karyotype have moderate to high pollen stainability, those involving A. batizocoi have zero pollen stainability and a very irregular PMC meiosis. Such infertile hybrids are the most likely to produce fertile, stable amphidiploids on doubling the chromosome complement. It is suggested that the cultivated peanut could have originated from such a sterile interspecific hybrid and on morphological and phytogeographic grounds the most likely genome donors are A. cardenasii (nomen nudum) and A. batizocoi of the species within section Arachis, which have been collected up to the present time.

Hybrid fertility and aphid resistance in the cross Nicotiana tabacum x N. gossei

Nicotiana gossei, a disease- and insect-resistance species, was hybridized with the cigar-wrapper variety of N. tabacum, ‘Florida 17’. The sterile interspecific hybrid was converted to a fertile amphiploid by treatment with colchicine. Of two methods of colchicine treatment one was more effective than the other. Certain differences in morphological details of shoot and leaf development were observed after colchicine treatment. Descriptions were also given of the relative fertility of the progenies resulting from reciprocal crosses of the recurrent parent Florida 17 by the amphiploid and by the sesquidiploid.

Anther Culture with a Sterile Interspecific Hybrid between Nicotiana tabacum and N. sylvestris

Anther culture was carried out using the sterile F1 hybrids between Nicotiana tabacum and N. sylvestris. Plantlets were developed from 3.4% of the anther. Some plants obtained showed new combinations of characters of the two parents, but they were supposed to be of the tapetum origin.