Hybridization and speciation in the genusDryopteris (Pteridophyta:Dryopteridaceae) on Pico Island in the Azores

The cytology of wild hybrids ofDryopteris from Pico, Azores has been examined. This has confirmed the parentage of the hybrids, and provided evidence to support the view that the tetraploid speciesD. crispifolia has evolved in the Azores from two diploid species,D. aemula andD. azorica, by hybridization and polyploidy.     

Adaptation to a Starch Environment and Regulation of alpha-Amylase in Drosophila

The adaptation to glucose and starch foods insix species, D. melanogaster, D.virilis, D. saltans, D. funebris,D. levanonensis and D. americana, wasstudied by measuring productivity. D.melanogaster and D. virilis adapted more to thestarch environment than to the glucose environment,while D. saltans adapted more to the glucoseenvironment than to the starch environment. D.funebris, D. levanonensis, and D. americana did not distinctlyadapt to either environment. In addition, the regulationof amylase in the six species was investigated bymeasuring the levels of amylase activity with glucoseand starch food environments. The levels of amylaseactivity in D. levanonensis and D.saltans were substantially low, indicating thatthese species cannot utilize starch as a carbon source.The starch-adapted species, D. melanogaster and D.virilis, showed higher levels of amylase activitywith the starch environment and higher inducibility.These results suggest that changing the regulation ofamylase is important for the adaptation to a starch environment inDrosophila.     

Genetic variation and hybridization in SwedishSchoenus (Cyperaceae)

Schoenus ferrugineus andS. nigricans have restricted distributions in Sweden and are almost exclusively confined to calcareous fen habitats. AtS. nigricans sites,S. ferrugineus is usually also present, and hybrids are frequently found. In this report, I used allozymes to estimate the amount of gene flow between the two species, and to compare the partitioning of genetic diversity in each of them. Thirteen loci were analysed at eight different enzyme systems. Seven loci were variable between or within the species. The two species had completely different alleles at two of the seven variable loci, whereas there was overlap at five loci. In all, 22 different alleles were found. Six of these alleles were confined toS. nigricans, and five alleles were confined toS. ferrugineus. Nei's genetic identity was 0.55.—InS. ferrugineus, three loci (23%) were polymorphic, and the average number of alleles per polymorphic locus was 2.0 (each polymorphic locus had two alleles). InS. nigricans, three loci (23%) were polymorphic, and the average number of alleles per polymorphic locus was 2.3.—The proportion of genetic diversity due to variation among sites (G _ST) was fairly similar in the two species, mean over loci = 0.12 inS. ferrugineus and 0.15 inS. nigricans. However, the proportion of genetic diversity due to variation among individuals within sites (G _IS) differed markedly between the two species, mean over loci = 0.54 inS. ferrugineus and 0.17 inS. nigricans. Accordingly, there was a much higher individual heterozygosity inS. nigricans than inS. ferrugineus. — Most hybrids were interpreted as F₁ hybrids. However, a small proportion, 0.5–1.6 %, were F_n hybrids or back-crosses.—On the Swedish mainland, all former occurrences ofS. nigricans are extinct, but viable hybrids are still present at a few sites in southernmost Sweden.     

Second generation experimental hybridizations in theEchinocereus ×lloydii complex (Cactaceae), and further documentation of dioecy inE. coccineus

Artificial second generation and backcross hybrids involvingEchinocereus dasyacanthus (Texas rainbow cactus),E. coccineus (a species of claret-cup cactus), andE. ×lloydii (Lloyd's hedgehog cactus) were examined for morphological and reproductive characteristics. The hybrids grew from seed to flower in about four years, and they exhibited high fertility as measured by pollen stainability, crossability, and fruit-set. A rare breeding system inCactaceae, dioecy, was further examined inE. coccineus, using field and controlled pollination tests. Functional dioecy was documented forE. coccineus in Trans-Pecos Texas. All results suggested thatE. ×lloydii in eastern Pecos County, Texas, is a complex and dynamic population probably comprising first and later generation hybrids, including backcross hybrids. The experimental hybridizations also tended to support the occurrence of introgression into both putative parental species.     

Electrophoretic variation of the apogamousDryopteris varia group (Dryopteridaceae)

A total of 35 electrophoretic variants were detected in four apogamous species of theDryopteris varia group: 14 variants inD. bissetiana triploids, 7 inD. sacrosancta triploids, 6 inD. varia triploids, 3 inD. pacifica triploids, and 5 in apogamous diploids. The variants differed from each other in various combinations of a few alleles which were common in some of the variants, suggesting recurrent origin.Dryopteris insularis var.insularis (2x) and var.chichisimensis (3x), other members of the group, were electrophoretically monomorphic, suggesting single origin. Banding patterns infer that triploids ofD. bissetiana andD. sacrosancta may have originated from hybrids whose parent is the sexualD. saxifraga, whereas this species was not involved in the origin of the other apogamous triploids and the apogamous diploids. The other parent was also discussed and we proposed a hypothesis that diploid offsprings fromD. pacifica triploids hybridized withD. saxifraga giving rise to triploids ofD. bissetiana andD. sacrosancta.     

A second spontaneous hybrid in the genusLeucaena (Leguminosae, Mimosoideae)

The detection of hybridity inLeucaena is discussed in relation to: (i) traditional criteria, (ii) molecular criteria and (iii) models to predict hybrid leaf morphology. Morphological, geographical and molecular evidence for the occurrence of interspecific hybrids betweenL. leucocephala andL. diversifolia in south-central Mexico, northern Guatemala, Jamaica, Dominican Republic, the Philippines and Papua New Guinea is presented. Predicted mean hybrid leaf trait values calculated from parent material are compared with data from putative hybrids and shown to be similar. The origin of these hybrids is discussed and shown to be the result of artificial sympatry resulting from indigenous, and recent exotic, domestication of the parent species. The hybrid is described asL. ×spontanea.     

Meiotic analyses ofCucumis hybrids and an evolutionary evaluation of the genusCucumis (Cucurbitaceae)

Meiosis in seven interspecificCucumis hybrids has been analysed i.a. inC. metuliferus ×C. zeyheri, where the parents belong to different sections. In the triploid hybrids a remarkably high number of trivalents has been found. Additional data from literature on geographical distribution, cucurbitacins, flavonoid patterns, isozymes, C-banding, genome size, DNA amount and chloroplast DNA are used to discuss species relationships and evolution. The African cross-compatible group is divided into theMyriocarpus subgroup with the diploid speciesC. africanus, C. myriocarpus subsp.leptodermis and subsp.myriocarpus, and theAnguria subgroup withC. anguria, C. dipsaceus, C. ficifolius, C. prophetarum, C. zeyheri and all polyploids (exceptC. heptadactylus). It is argued that the Asian subg.Melo with x = 7 is derived from the African subg.Cucumis with x = 12; the latter contains all the polyploid species and has the most common basic chromosome number of theCucurbitaceae. This phylogenetic advance is interpreted with concepts of the quantum model of evolution.     

Experimental interspecific hybridization in Daphnia

Hybridization is a common phenomenon in Daphnia (Cladocera; Anomopoda); interspecific hybrids have been found between several species and hybrids are found in many European lakes. Although much information on the morphology, ecology and genetics of hybrids is available, little is known about the level of reproductive isolation among species or about the relative fitness of hybrids and parental species. In order to facilitate studies on differentiation and speciation processes and comparative experimental studies on hybrids and recombinant genotypes, we present the first successful laboratory crossing experiments of two different Daphnia species, D. galeata and D. cucullata. Males and sexual females from two D. galeata and two D. cucullata clones were reciprocally crossed, juveniles hatched from resting eggs and reared until maturity. Hatching and juvenile survival rates of hybrids were relatively low (12.1% and 24%, respectively). D. galeata and D. cucullata clones vary in their level of successful interspecific matings and in the number of subsequent offspring. In general, hybrid crosses between D. cucullata females and D. galeata males were more successful than reciprocal crosses.     

Distribution of parental DNA markers inEncelia virginensis (Asteraceae: Heliantheae), a diploid species of putative hybrid origin

Morphological, geographical and ecological evidence suggests thatEncelia virginensis is a true-breeding diploid species derived from hybrids ofE. actoni andE. frutescens. To test this hypothesis, we examined the chloroplast and nuclear DNA of severalEncelia species. PCR amplification targeted three separate regions of chloroplast DNA:trnK-2621/trnK-11,rbcL/ORF106, andpsbA3/TrnI-51, which amplify 2600bp, 3300bp and 3200bp fragments respectively. Restriction fragment analysis of chloroplast DNA revealed no variation that could be used to discriminate between the parent species. A RAPD analysis using 109 dekamer primers was used to analyze the nuclear genome.Encelia actoni andE. frutescens were distinguished by several high-frequency RAPD markers. In populations ofE. virginensis, these markers were detected in varying proportions, and no unique markers were found. Evidence from the nuclear genome supports the hypothesis thatE. virginensis is of hybrid origin. ThatE. virginensis may have arisen by normal divergent speciation followed by later introgression remains a possibility, however, and is not formally ruled out here. Diploid hybrid speciation inEncelia differs from other documented cases in that there are no discernible chromosome differences between the species, and all interspecific hybrids are fully fertile. In addition, apparent ecological selection against backcross progeny provides an external barrier to reproduction between F₁ progeny and the parental species. These characteristics suggest that hybrid speciation inEncelia may represent an alternative model for homoploid hybrid speciation involving external reproductive barriers. In particular, this may be the case for other proposed diploid hybrid taxa that also exhibit little chromosomal differentiation and have fertile F₁s.     

Intergeneric hybridization between Diplotaxis siifolia,a wild species and crop brassicas

Summary Attempts were made to obtain intergeneric hybrids between Diplotaxis siifolia, a wild species, and cultivars of Brassica (B. campestris, B. juncea, and B. napus). The crosses showed unilateral incompatibility. When the wild species was used as female parent, pollen germination and pollen tube growth were normal, but hybrid seeds aborted due to post-fertilization barriers. Reciprocal crosses (cultivars as female parent) showed strong pre-fertilization barriers; although pollen grains showed germination, pollen tubes failed to enter the stigma. Hybrids were realized in two of the crosses, D. siifolia x B. juncea and D. siifolia x B. napus, through ovary culture. The hybrids were multiplied in vitro by multiplication of axillary shoots, or somatic embryogenesis. Detailed studies were carried out on the hybrid D. siifolia x B. juncea. F₁ hybrids had shrivelled anthers and were pollen sterile. Amphiploids of this hybrid showed 60% pollen fertility and produced seeds upon self-pollination as well as backcross pollination with the pollen of B. juncea.     

Characterization ofGandalf, a new inverted-repeat transposable element ofDrosophila koepferae

The cloning and characterization ofGandalf, a new DNA-transposing mobile element obtained from theDrosophila koepferae (repleta group) genome is described. A fragment ofGandalf was found in a middle repetitive clone that shows variable chromosomal localization. Restriction, Southern blot, PCR and sequencing analyses have shown that mostGandalf copies are about 1 kb long, are flanked by 12 by inverted terminal repeats and contain subterminal repetitive regions on both sides of the element. As with other elements of the DNA-transposing type (known as the Ac family), theGandalf element generates 8 by direct duplications at the insertion point. Coding region analysis has shown that the longer open reading frame found inGandalf copies could encode part of a protein. However, whether or not the 1 kb copies of the element are actually the active transposons remains to be elucidated.Gandalf shows a very low copy number inD. buzzatii, a sibling species ofD. koepferae. An attempt to induce interspecific hybrid dysgenesis in hybrids of these two species has been unsuccessful.     

Genetic variation and putative hybridization inSalix alba andS. fragilis (Salicaceae): Evidence from allozyme data

Genetic variability was estimated by enzyme electrophoresis in 239 Belgian clones from theSalix alba-S. fragilis complex. This morphological complex suggested a high frequency of hybrids. To test this hypothesis, the clones were pooled as a single co-adapted species complex and secondly as belonging to either species, i.e. beingS. alba-like orS. fragilis-like. The standard genetic variability measures showed higher values for the complex than for the separate species. However, the observed mean heterozygosity was lower in the putative hybrid complex than for each of the species separately. The fixation indices were more variable at the species level and indicated that mostS. fragilis locations appeared fixed forlap-1, whereas no fixation occured inS. alba locations. Averaged at the regional (i.e. catchment) level, this difference between the two species remained and values ranged from 0.457 to 0.617 inS. alba and were much higher, fixed homozygous or monomorph inS. fragilis. Hierarchical F-statistics revealed that most of the differentiation occured at the lower levels of localities and tributaries and that there was no further differentiation between catchments. Tributaries which are 10–25km in length were proposed as the most likely entities for further examination of putative hybridization and events of allelic fixations. By considering the two abovementioned approaches of data input, it could be suggested that most of the allozyme differentiation was between the species and less between the regions.     

An analysis of the B genome speciesArachis batizocoi (Fabaceae)

Arachis batizocoi Krap. & Greg. is a suggested B genome donor to the cultivated peanut,A. hypogaea L. Until recently, only one accession of this species was available in U.S.A. germplasm collections for analyses and species variability had not been documented. The objective of this study was to determine the intraspecific variability ofA. batizocoi to better understand phylogenetic relationships in sect.Arachis. Five accessions of the species were used for morphological and cytological studies and then F₁ intraspecific hybrids analyzed. Some variation was observed among accessions—for example, differences in seed size, plant height and branch length. The somatic chromosomes of accessions 9484, 30079, and 30082 were nearly identical, whereas, the karyotypes of accessions 30081 and 30097 have several distinct differences. For example, 30081 had significantly more asymmetrical chromosomes 2 and 6 and more median chromosomes 7 and 10, and 30097 had significantly more asymmetrical chromosomes 3 and 10 and more median chromosomes 1 and 5 than accessions 9484, 30079, and 30082. All F₁ hybrids among accessions were highly fertile. Meiotic observations indicated that hybrids among accessions 9484, 30079, or 30082 had mostly bivalents. However, quadrivalents were observed when either 30081 or 30097 was crossed with the above three accessions and 30081 × 30097 had quadrivalents, hexavalents and octavalents. The presence of translocations is the most likely cause of multivalent formation inA. batizocoi hybrids. Cytological evolution via translocations has apparently been an important mechanism for differentiation in the species.Paper No. 12382 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643.     

Daphnia Galeata × D. Longispina Hybrids in Western Norway

We describe the occurrence of D. galeata×longispina hybrids in two lakes of western Norway. Parental species and interspecific hybrids were characterised by both nuclear and mitochondrial molecular markers. In one of the populations, hybrids were shown to dominate the population over several years. A few individuals in both populations were probably not F1 hybrids, but possibly backcrosses or F2 hybrids. Most (possibly all) F1 hybrids were of D. galeata maternal origin. In addition, interspecific hybrids could be identified based on morphological characters, which were intermediate between the parental species. Interspecific hybridisation between these two species is remarkable, since they are distantly related.     

Production of wide hybrids and backcross progenies between Diplotaxis erucoides and crop brassicas

Intergeneric hybrids were produced between D. erucoides (), a wild species, and four cultivated species of Brassica, B. campestris, B. juncea, B. napus and B. oleracea, through embryo rescue. The hybrid nature of these plants was confirmed through morphological and cytological studies. Backcross pollinations with the pollen of the respective cultivars yielded BC progenies in the hybrids D. erucoides x B. juncea and D. erucoides x B. napus but not in D. erucoides x B. campestris and D. erucoides x B. oleracea. The hybrid D. erucoides x B. campestris was also used as a bridge species and crossed with B. juncea to raise the hybrid and backcross progenies. F₂ progenies were more amenable than f₁ hybrids for raising backcross progenies. Although D. erucoides is considered to be a close relative of B. campestris and B. oleracea, incompatibility barriers of this species with different cultivars do not reflect this relationship.     

Quantitative comparison of foliage development amongDryopteris monticola,D. tokyoensis and a putative hybrid,D. kominatoensis in Northern Japan

Developmental leaf architecture was quantitatively described in terms of measurements of various parameters on leaf blade from different size of sporophytes inDryopteris monticola, D. tokyoensis and a putative hybrid,D. kominatoensis in the natural site of Hokkaido, to compare the ontogenetic differentiation in foliage structure among allied ferns. The morphological stage of leaf and sporophyte was tentatively quantified by the number of midrib branches of the leaf (NV, number of veins), which exhibited a significant correlation to the leaf-shape complexity from a circle (DI=marginal length/2×(3.14×square)^1/2) of leaf blade. D. kominatoensis showed intermediate values between others in following characters; DI increase, maximum NV (also blade length), maximum number of costa branches of pinnae (NVMP), number of costa branches of the lowest pinna (NVLP), difference between NVMP and NVLP (NVMP-NVLP), during heteroblastic leaf development. A larger number of leaves per sporophyte was found inD. kominatoensis than in others. The fertility rate (%) and initiation of fertility (IF) in the relative developmental stage (RDS) ofD. kominatoensis shifted to that ofD. tokyoensis, while the order of pinnae with NVMP shifted to that ofD. monticola. Even in the intermediate characters inD. kominatoensis, slight shifts in characters to those of putative parents were found during heteroblastic leaf development. Contribution No. 3145 from the Institute of Low Temperature Science, Hokkaido University.     

An experimental study of evolution in progress: Clines for quantitative traits in colonizing and Palearctic populations ofDrosophila

Summary Drosophila subobscura has recently colonized the American continent and is an excellent model for studying evolution in action. Previous analyses have shown that these colonizing populations have significant latitudinal clines for the frequencies of some chromosomal arrangements that parallel those clines found in the Old World. These results strongly suggest that this polymorphism is adaptive. In the present study, significant latitudinal clines for continuous morphometric variables (flies are larger in the north) have been detected in Old World populations ofD. subobscura. The adaptive nature of these clines is reinforced by the fact that parallel latitudinal clines for body size have also been detected inDrosophila obscura, a closely related sympatric species, as well as previously in otherDrosophila. On the other hand, no significant latitudinal clines for continuous morphometric traits, not even when using an overall size index, have been detected in colonizing populations ofD. subobscura. This is a rather surprising result given the number of generations that have elapsed since the species was detected in America and given that significant clines in chromosomal inversions are already established. Thus, the adaptive response of quantitative morphometric variables is not as rapid as that found for chromosomal inversions. Nevertheless, canonical correlation analysis suggests that significant latitudinal clines for body size might soon be detected in the American continent. The results obtained here are included in a projected time series with the aim of documenting size evolution in action.     

Crossing experiments of lettuce cultivars and species (Lactuca sect.Lactuca,Compositae)

The degree of relationships withinLactuca sativa and three wild relativesL. serriola, L. saligna, andL. virosa was studied by observing the performance, vigour and fertility of the F 1 hybrids obtained from crosses made in and between the four species. The crosses ofL. saligna ×L. virosa and the reciprocal crosses produced no hybrids.L. saligna andL. virosa are the least related of the four species.L. sativa ×L. serriola and the reciprocal crosses were successful and produced fertile hybrids These two species are genetically very closely related.L. saligna is known to produce, as a female parent, hybrids withL. sativa andL. serriola. Now the reciprocal cross was successful for the first time, so the unability to obtain hybrids in the past was based on the choice of accessions and not caused by unilateral incompatibility.L. virosa ×L. sativa and the reciprocal combination produced hybrids. The combinationL. serriola ×L. virosa produced hybrids with very limited fertility. In contrast to earlier reports (sterile hybrids) one combination of the reciprocal cross too produced hybrids with very limited fertility.—Some of theL. saligna ×L. sativa (and reciprocal) hybrids were found to look strikingly likeL. serriola. This adds evidence for the descent ofL. serriola andL. sativa:L. saligna also made part of the ancestral complex of the cultivated lettuce.     

Hybrid lethal systems in theDrosophila melanogaster species complex

Lethal phases of the hybrids betweenDrosophila melanogaster and its sibling species,D. simulans are classified into three types: (1) embryonic lethality in hybrids carryingD. simulans cytoplasm andD. melanogaster X chromosome, (2) larval lethality in hybrids not carryingD. simulans X, and (3) temperature-sensitive pupal lethality in hybrids carryingD. simulans X. The same lethal phases are also observed when either of the two other sibling species,D. mauritiana orD. sechellia, is employed for hybridization withD. melanogaster. Here, we describe genetic analyses of each hybrid lethality, and demonstrate that these three types of lethality are independent phenomena. We then propose two models to interpret the mechanisms of each hybrid lethality. The first model is a modification of the conventional X/autosome imbalance hypothesis assuming a lethal gene and a suppressor gene are involved in the larval lethality, while the second model is for embryonic lethality assuming an interaction between a maternal-effect lethal gene and a suppressor gene.     

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.