Maternal inheritance of mitochondrial genomes and complex inheritance of chloroplast genomes in Actinidia Lind.: evidences from interspecific crosses

The inheritance pattern of chloroplast and mitochondria is a critical determinant in studying plant phylogenetics, biogeography and hybridization. To better understand chloroplast and mitochondrial inheritance patterns in Actinidia (traditionally called kiwifruit), we performed 11 artificial interspecific crosses and studied the ploidy levels, morphology, and sequence polymorphisms of chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) of parents and progenies. Sequence analysis showed that the mtDNA haplotypes of F1 hybrids entirely matched those of the female parents, indicating strictly maternal inheritance of Actinidia mtDNA. However, the cpDNA haplotypes of F1 hybrids, which were predominantly derived from the male parent (9 crosses), could also originate from the mother (1 cross) or both parents (1 cross), demonstrating paternal, maternal, and biparental inheritance of Actinidia cpDNA. The inheritance patterns of the cpDNA in Actinidia hybrids differed according to the species and genotypes chosen to be the parents, rather than the ploidy levels of the parent selected. The multiple inheritance modes of Actinidia cpDNA contradicted the strictly paternal inheritance patterns observed in previous studies, and provided new insights into the use of cpDNA markers in studies of phylogenetics, biogeography and introgression in Actinidia and other angiosperms.     

Next-generation sequencing analysis of lager brewing yeast strains reveals the evolutionary history of interspecies hybridization

The lager beer yeast Saccharomyces pastorianus is considered an allopolyploid hybrid species between S. cerevisiae and S. eubayanus. Many S. pastorianus strains have been isolated and classified into two groups according to geographical origin, but this classification remains controversial. Hybridization analyses and partial PCR-based sequence data have indicated a separate origin of these two groups, whereas a recent intertranslocation analysis suggested a single origin. To clarify the evolutionary history of this species, we analysed 10 S. pastorianus strains and the S. eubayanus type strain as a likely parent by Illumina next-generation sequencing. In addition to assembling the genomes of five of the strains, we obtained information on interchromosomal translocation, ploidy, and single-nucleotide variants (SNVs). Collectively, these results indicated that the two groups of strains share S. cerevisiae haploid chromosomes. We therefore conclude that both groups of S. pastorianus strains share at least one interspecific hybridization event and originated from a common parental species and that differences in ploidy and SNVs between the groups can be explained by chromosomal deletion or loss of heterozygosity.     

Karyological and flow cytometric evidence of triploid specimens in Bufo viridis (Amphibia Anura)

Karyological and flow cytometric (FCM) analyses were performed on a group of 14 green toads of the Bufo viridis species from seven Eurasian populations. Both approaches gave concordant results concerning the DNA ploidy level. All the populations examined were represented exclusively by diploid or tetraploid specimens, except one, where triploids were found. Results evidenced an interpopulation variability in DNA content against the same ploidy level, as well as an unusually high number of triploids in a particular reproductive place. The origin of polyploidy and the presence and persistence of a high number of triploids in a particular population are discussed.     

Homoploid hybrid speciation and genome evolution via chromosome sorting

Genomes of numerous diploid plant and animal species possess traces of interspecific crosses, and many researches consider them as support for homoploid hybrid speciation (HHS), a process by which a new reproductively isolated species arises through hybridization and combination of parts of the parental genomes, but without an increase in ploidy. However, convincing evidence for a creative role of hybridization in the origin of reproductive isolation between hybrid and parental forms is extremely limited. Here, through studying Agrodiaetus butterflies, we provide proof of a previously unknown mode of HHS based on the formation of post-zygotic reproductive isolation via hybridization of chromosomally divergent parental species and subsequent fixation of a novel combination of chromosome fusions/fissions in hybrid descendants. We show that meiotic segregation, operating in the hybrid lineage, resulted in the formation of a new diploid genome, drastically rearranged in terms of chromosome number. We also demonstrate that during the heterozygous stage of the hybrid species formation, recombination was limited between rearranged chromosomes of different parental origin, representing evidence that the reproductive isolation was a direct consequence of hybridization.     

MITOCHONDRIAL-DNA ANALYSES AND THE ORIGIN AND RELATIVE AGE OF PARTHENOGENETIC LIZARDS (GENUS CNEMIDOPHORUS). II. C. NEOMEXICANUS AND THE C. TESSELATUS COMPLEX

Restriction-endonuclease analyses of mitochondrial DNAs from all six color-pattern classes (A–F) of the parthenogenetic lizard Cnemidophorus tesselatus yield estimates of nucleotide divergence that are extremely low (π = 0.06%). In digests of 75 C. tesselatus mtDNAs with 20 different restriction enzymes, only four cleavage-site differences were noted, three of which were found only in pattern class F. The near-identity of these mitochondrial DNAs with those from C. tigris marmoratus shows unequivocally that C. t. marmoratus was the species to which the maternal parent(s) of all C. tesselatus belonged. Mitochondrial-DNA analyses of another unisexual species, C. neomexicanus, led to the same conclusion. Mitochondrial DNAs from 96 individuals of these three species were extensively analyzed for cleavage-site differences; only 13 were found. The low interspecific sequence diversity found within C. neomexicanus and the C. tesselatus complex suggests a recent origin for both. Based on diversity data for mitochondrial DNA and allozymes, we estimate that a minimum of two hybridizations were required to produce all diploid C. tesselatus (C–F), followed by at least two more to generate the triploids (A and B). These data and those presented in the two accompanying papers indicate that events leading to parthenogenesis in Cnemidophorus are rare and strengthen the hypothesis that interspecific hybridization is a necessary, causal event in its establishment.     

Mitochondrial DNA variation in cultivated and wild potato species (Solanum spp.).

The European cultivated potato, Solanum tuberosum subsp. tuberosum, has 6 related cultivated species and more than 200 wild relatives. In Solanum spp., studies of cytoplasmic organelles have been mainly confined to the plastid DNA composition of cultivated and wild species. In this study, 53 genotypes of 30 potato species belonging to the subsections Estolonifera and Potatoe, 2 tomato species, and a black nightshade genotype were examined using PCR markers to evaluate mitochondrial DNA diversity and assess whether mtDNA variability was correlated with series classification, geographical origin, ploidy, and endosperm balance number (EBN). The markers used revealed interspecific mtDNA variability in Solanum spp. and identified 13 different haplotypes. Intraspecific variability was also observed in a few species and genomic regions. Cluster analysis allowed arrangement of the 13 haplotypes into 7 subgroups, and statistical association tests showed significant relationships between mitochondrial patterns detected by molecular analysis and ploidy, EBN, and geographical origin. On the whole, the evolutionary patterns for the genomic regions analyzed reflected the species relationships established on the basis of morphological and molecular (nuclear and plastidial DNA) data. The mtDNA variability shown is also important for better characterization of genetic resources for potato breeding.     

利用微卫星遗传标记探讨达氏鳇的多倍体倍性

鲟形目(Acipenseriformes)鱼类是一类多倍体起源的鱼类,种问易于杂交,且由于存在大量微型染色体,其染色体数目和倍性也难于确定.到目前为止包括达氏鳇(Huso dauricus)在内的一些鲟鱼物种基因组大小和染色体倍性仍旧存在争议.本实验采用微卫星遗传标记技术,通过观察9个微卫星位点的等位基因数目,发现达氏鳇在大部分位点中显示的倍性大于二倍性,同时利用多倍体分析软件POLYSAT推断了26尾达氏鳇的个体倍性.结果表明,26尾达氏鳇中有20尾显示为六倍体,占总数的76.92％;4尾显示为八倍体,占总数的15.38％;2尾显示为四倍体,占总数的7.79％.因此,我们认为达氏鳇应该为八倍体物种,结果中出现的四倍体和六倍体个体,是由于这些个体在我们选取的微卫星位点中存在部分纯合等位基因,导致了多数个体显示为六倍体.这一结论与近年来支持达氏鳇为进化性八倍体物种的研究结果一致.     

Dynamic Formation of Asexual Diploid and Polyploid Lineages: Multilocus Analysis of Cobitis Reveals the Mechanisms Maintaining the Diversity of Clones

Given the hybrid genomic constitutions and increased ploidy of many asexual animals, the identification of processes governing the origin and maintenance of clonal diversity provides useful information about the evolutionary consequences of interspecific hybridization, asexuality and polyploidy. In order to understand the processes driving observed diversity of biotypes and clones in the Cobitis taenia hybrid complex, we performed fine-scale genetic analysis of Central European hybrid zone between two sexual species using microsatellite genotyping and mtDNA sequencing. We found that the hybrid zone is populated by an assemblage of clonally (gynogenetically) reproducing di-, tri- and tetraploid hybrid lineages and that successful clones, which are able of spatial expansion, recruit from two ploidy levels, i.e. diploid and triploid. We further compared the distribution of observed estimates of clonal ages to theoretical distributions simulated under various assumptions and showed that new clones are most likely continuously recruited from ancestral populations. This suggests that the clonal diversity is maintained by dynamic equilibrium between origination and extinction of clonal lineages. On the other hand, an interclonal selection is implied by nonrandom spatial distribution of individual clones with respect to the coexisting sexual species. Importantly, there was no evidence for sexually reproducing hybrids or clonally reproducing non-hybrid forms. Together with previous successful laboratory synthesis of clonal Cobitis hybrids, our data thus provide the most compelling evidence that 1) the origin of asexuality is causally linked to interspecific hybridization; 2) successful establishment of clones is not restricted to one specific ploidy level and 3) the initiation of clonality and polyploidy may be dynamic and continuous in asexual complexes.     

Elucidating polyploidization of bermudagrasses as assessed by organelle and nuclear DNA markers

Clarification of relationships among ploidy series of Cynodon accessions could be beneficial to bermudagrass breeding programs, and would enhance our understanding of the evolutionary biology of this warm season grass species. This study was initiated to elucidate polyploidization among Cynodon accessions with different ploidy series collected from Turkey based on chloroplast and nuclear DNA. Forty Cynodon accessions including 7 diploids, 3 triploids, 10 tetraploids, 11 pentaploids, and 9 hexaploids were analyzed using chloroplast DNA restriction fragment-length polymorphism (cpDNA RFLP), chloroplast DNA simple sequence repeat (cpDNA SSR), and nuclear DNA markers based on neighbor-joining (NJ) and principle component analyses (PCA). All three-marker systems with two statistical algorithms clustered the diploids apart from the other ploidy levels. Assuming autopolyploidy, spontaneous polyploidization followed by rapid diversification among the higher ploidy levels than the diploids is likely in Cynodon's evolution. Few tetraploid and hexaploid accessions were clustered with or closely to the group of diploids, supporting the hypothesis above. Eleven haplotypes as estimated by cpDNA RFLP and SSR markers were detected. This study indicated that the diploids had different organelle genome from the rest of the ploidy series and provided valuable insight into relationships among ploidy series of Cynodon accessions based on cp and nuclear DNAs.     

Differentiating diploid and triploid individuals using single nucleotide polymorphisms genotyped by amplicon sequencing

Triploidy can occur naturally or be induced in fish and shellfish during artificial propagation in order to produce sterile individuals. Fisheries managers often stock these sterile triploids as a means of improving angling opportunities without risking unwanted reproduction of the stocked fish. Additionally, the rearing of all‐triploid individuals has been suggested as a means to reduce the possibility of escaped aquaculture fish interbreeding with wild populations. Efficient means of determining if an individual is triploid or diploid are therefore needed both to monitor the efficacy of triploidy‐inducing treatments and, when sampling fish from a body of water that has a mixture of diploids and triploids, to determine the ploidy of a fish prior to further analyses. Currently, ploidy is regularly measured through flow cytometry, but this technique typically utilizes a fresh blood sample. This study presents an alternative, cost‐effective method of determining ploidy by analysing amplicon‐sequencing data for biallelic single‐nucleotide polymorphisms (SNPs). For each sample, heterozygous genotypes are identified and the likelihoods of diploidy and triploidy are calculated based on the read counts for each allele. The accuracy of this method is demonstrated using triploid and diploid brook trout (Salvelinus fontinalis) genotyped with a panel of 234 SNPs and Chinook salmon (Oncorhynchus tshawytscha) genotyped with a panel of 298 SNPs following the GT‐seq methodology of amplicon sequencing.     

Interspecific hybridization in the genus Hieracium s. str.: evidence for bidirectional gene flow and spontaneous allopolyploidization

Although reticulation has indisputably played an important role in the evolutionary history of the genus Hieracium s. str. (Asteraceae), convincingly documented cases of recent interspecific hybridization are very rare. Here we report combined evidence on recent hybridization between two diploid species, Hieracium alpinum and H. transsilvanicum. The hybrid origin of the plants from the Romanian Eastern Carpathians was supported by additive patterns of nuclear ribosomal DNA polymorphism (ITS), an intermediate position of hybrid plants in principal coordinate analysis based on amplified fragment length polymorphism phenotypes (AFLP), and additivity at one allozyme locus. Flow cytometric analyses and chromosome counting showed that two hybrids were diploid (2n ~ 2x ~ 18) while one was surprisingly tetraploid (2n = 4x = 36). To our knowledge, this is the first record of spontaneous polyploidization following interspecific crossing in the genus. Allozyme data, especially the presence of unbalanced heterozygosity at one locus, suggest the origin of this tetraploid via a triploid bridge with subsequent backcrossing to H. alpinum. According to PCR-RFLP analyses of the trnT-trnL intergenic spacer, all H. ×krasani hybrids examined had the H. alpinum haplotype while H. transsilvanicum served as a pollen donor. The hybrids occurred at the locality with abundant H. alpinum plants where paternal H. transsilvanicum was missing. Previously reported instances of interspecific hybridization between the same parental taxa showed an opposite direction of crossing and relative abundance of parental taxa. This suggests that the direction of hybridization might be influenced by the frequency of parental taxa at the locality.     

Origin and Genetic Diversity of Diploid Parthenogenetic Artemia in Eurasia

There is wide interest in understanding how genetic diversity is generated and maintained in parthenogenetic lineages, as it will help clarify the debate of the evolution and maintenance of sexual reproduction. There are three mechanisms that can be responsible for the generation of genetic diversity of parthenogenetic lineages: contagious parthenogenesis, repeated hybridization and microorganism infections (e.g. Wolbachia). Brine shrimps of the genus Artemia (Crustacea, Branchiopoda, Anostraca) are a good model system to investigate evolutionary transitions between reproductive systems as they include sexual species and lineages of obligate parthenogenetic populations of different ploidy level, which often co-occur. Diploid parthenogenetic lineages produce occasional fully functional rare males, interspecific hybridization is known to occur, but the mechanisms of origin of asexual lineages are not completely understood. Here we sequenced and analysed fragments of one mitochondrial and two nuclear genes from an extensive set of populations of diploid parthenogenetic Artemia and sexual species from Central and East Asia to investigate the evolutionary origin of diploid parthenogenetic Artemia, and geographic origin of the parental taxa. Our results indicate that there are at least two, possibly three independent and recent maternal origins of parthenogenetic lineages, related to A. urmiana and Artemia sp. from Kazakhstan, but that the nuclear genes are very closely related in all the sexual species and parthenogegetic lineages except for A. sinica, who presumable took no part on the origin of diploid parthenogenetic strains. Our data cannot rule out either hybridization between any of the very closely related Asiatic sexual species or rare events of contagious parthenogenesis via rare males as the contributing mechanisms to the generation of genetic diversity in diploid parthenogenetic Artemia lineages.     

Recent invasion and low level of divergence between diploid and triploid forms of <Emphasis Type="Italic">Carassius auratus</Emphasis> complex in Croatia

Carassius auratus is an invasive species in European waters, comprising a complex of diploid and polyploid forms with different modes of reproduction. However, the evolutionary history and relationships between the diploids and polyploids are still unresolved. In this study, 51.5% diploids and 48.5% triploids, including four triploid males, were discovered among the 363 individuals sampled in Croatia. We used eight microsatellite loci and mitochondrial displacement loop sequences to analyze the structure and origin of populations; and to attempt to infer the evolutionary history of the two different forms in Croatia. Microsatellite analyses revealed high allelic and clonal diversity, corroborating that high propagule vectors can compensate for the negative effects of genetic bottlenecks in successful invasive species. The absence of significant population structuring confirmed recent origin and rapid spreading of populations. No evidence was found for the existence of native European populations. Distances between individuals using both nuclear and mtDNA markers revealed the absence of substantial clustering on the ploidy level, while the split between the different ploidies on population level was only partial, suggesting that the reproductive isolation between the two forms is either of a very recent origin, or that there exists uni-, or bidirectional gene flow between the diploid and triploid forms.     

Analysis of genome-wide linkage disequilibrium in the highly polyploid sugarcane

Linkage disequilibrium (LD) in crops, established by domestication and early breeding, can be a valuable basis for mapping the genome. We undertook an assessment of LD in sugarcane (Saccharum spp), characterized by one of the most complex crop genomes, with its high ploidy level (>or=8) and chromosome number (>100) as well as its interspecific origin. Using AFLP markers, we surveyed 1,537 polymorphisms among 72 modern sugarcane cultivars. We exploited information from available genetic maps to determine a relevant statistical threshold that discriminates marker associations due to linkage from other associations. LD is very common among closely linked markers and steadily decreases within a 0-30 cM window. Many instances of linked markers cannot be recognized due to the confounding effect of polyploidy. However, LD within a sample of cultivars appears as efficient as linkage analysis within a controlled progeny in terms of assigning markers to cosegregation groups. Saturating the genome coverage remains a challenge, but applying LD-based mapping within breeding programs will considerably speed up the localization of genes controlling important traits by making use of phenotypic information produced in the course of selection.     

Phylogenetic origin of limes and lemons revealed by cytoplasmic and nuclear markers

Background and Aims The origin of limes and lemons has been a source of conflicting taxonomic opinions. Biochemical studies, numerical taxonomy and recent molecular studies suggested that cultivated Citrus species result from interspecific hybridization between four basic taxa (C. reticulata, C. maxima, C. medica and C. micrantha). However, the origin of most lemons and limes remains controversial or unknown. The aim of this study was to perform extended analyses of the diversity, genetic structure and origin of limes and lemons.Methods The study was based on 133 Citrus accessions. It combined maternal phylogeny studies based on mitochondrial and chloroplastic markers, and nuclear structure analysis based on the evaluation of ploidy level and the use of 123 markers, including 73 basic taxa diagnostic single nucleotide polymorphism (SNP) and indel markers.Key Results The lime and lemon horticultural group appears to be highly polymorphic, with diploid, triploid and tetraploid varieties, and to result from many independent reticulation events which defined the sub-groups. Maternal phylogeny involves four cytoplasmic types out of the six encountered in the Citrus genus. All lime and lemon accessions were highly heterozygous, with interspecific admixture of two, three and even the four ancestral taxa genomes. Molecular polymorphism between varieties of the same sub-group was very low.Conclusions Citrus medica contributed to all limes and lemons and was the direct male parent for the main sub-groups in combination with C. micrantha or close papeda species (for C. aurata, C. excelsa, C. macrophylla and C. aurantifolia – ‘Mexican’ lime types of Tanaka’s taxa), C. reticulata (for C. limonia, C. karna and C. jambhiri varieties of Tanaka’s taxa, including popular citrus rootstocks such as ‘Rangpur’ lime, ‘Volkamer’ and ‘Rough’ lemons), C. aurantium (for C. limetta and C. limon – yellow lemon types – varieties of Tanaka’s taxa) or the C. maxima × C. reticulata hybrid (for C. limettioides – ‘Palestine sweet’ lime types – and C. meyeri). Among triploid limes, C. latifolia accessions (‘Tahiti’ and ‘Persian’ lime types) result from the fertilization of a haploid ovule of C. limon by a diploid gamete of C. aurantifolia, while C. aurantifolia triploid accessions (‘Tanepao’ lime types and ‘Madagascar’ lemon) probably result from an interspecific backcross (a diploid ovule of C. aurantifolia fertilized by C. medica). As limes and lemons were vegetatively propagated (apomixis, horticultural practices) the intra-sub-group phenotypic diversity results from asexual variations.     

The Causes and Evolutionary Consequences of Mixed Singing in Two Hybridizing Songbird Species (Luscinia spp.)

Bird song plays an important role in the establishment and maintenance of prezygotic reproductive barriers. When two closely related species come into secondary contact, song convergence caused by acquisition of heterospecific songs into the birds’ repertoires is often observed. The proximate mechanisms responsible for such mixed singing, and its effect on the speciation process, are poorly understood. We used a combination of genetic and bioacoustic analyses to test whether mixed singing observed in the secondary contact zone of two passerine birds, the Thrush Nightingale (Luscinia luscinia) and the Common Nightingale (L. megarhynchos), is caused by introgressive hybridization. We analysed song recordings of both species from allopatric and sympatric populations together with genotype data from one mitochondrial and seven nuclear loci. Semi-automated comparisons of our recordings with an extensive catalogue of Common Nightingale song types confirmed that most of the analysed sympatric Thrush Nightingale males were ‘mixed singers’ that use heterospecific song types in their repertoires. None of these ‘mixed singers’ possessed any alleles introgressed from the Common Nightingale, suggesting that they were not backcross hybrids. We also analysed songs of five individuals with intermediate phenotype, which were identified as F₁ hybrids between the Thrush Nightingale female and the Common Nightingale male by genetic analysis. Songs of three of these hybrids corresponded to the paternal species (Common Nightingale) but the remaining two sung a mixed song. Our results suggest that although hybridization might increase the tendency for learning songs from both parental species, interspecific cultural transmission is the major proximate mechanism explaining the occurrence of mixed singers among the sympatric Thrush Nightingales. We also provide evidence that mixed singing does not substantially increase the rate of interspecific hybridization and discuss the possible adaptive value of this phenomenon in nightingales.     

The role of triploids in the origin and evolution of polyploids of <Emphasis Type="Italic">Turnera sidoides</Emphasis> complex (Passifloraceae,Turneroideae)

Triploids can play an important role in polyploid evolution. However, their frequent sterility is an obstacle for the origin and establishment of neotetraploids. Here we analyzed the microsporogenesis of triploids (x?=?7) and the crossability among cytotypes of Turnera sidoides, aiming to test the impact of triploids on the origin and demographic establishment of tetraploids in natural populations. Triploids of T. sidoides exhibit irregular meiotic behavior. The high frequency of monovalents and of trivalents with non-convergent orientations results in unbalanced and/or non-viable male gametes. In spite of abnormalities in chromosome pairing and unbalanced chromosome segregation, triploids are not completely sterile and yielded up to 67% of viable pollen. Triploids that originated by the fusion of 2n?×?n gametes of the same taxon showed more regular meiotic behavior and higher fertility than triploids from the contact zone of diploids and tetraploids or triploids of hybrid origin. The reproductive isolation of T. sidoides cytotypes of different ploidy level is not strict and the ‘triploid block’ may be overcome occasionally. Triploids of T. sidoides produce diploid and triploid progeny suggesting that new generations of polyploids could originate from crosses between triploids or from backcrosses with diploids. The capability of T. sidoides to multiply asexually by rhizomes, would enhance the likelihood that a low frequency of neopolyploids can be originated and maintained in natural populations of T. sidoides.     

Evolution of cryptic gene pools in Hypericum perforatum: the influence of reproductive system and gene flow

Background and Aims Hypericum perforatum

(St. John''s wort) is a widespread Eurasian perennial plant species with remarkable variation in its morphology, ploidy and breeding system, which ranges from sex to apomixis. Here, hypotheses on the evolutionary origin of St. John''s wort are tested and contrasted with the subsequent history of interspecific gene flow.

Methods

Extensive field collections were analysed for quantitative morphological variation, ploidy, chromosome numbers and genetic diversity using nuclear (amplified fragment length polymorphism) and plastid (trnL-trnF) markers. The mode of reproduction was analysed by FCSS (flow cytometric seed screen).

Key Results

It is demonstrated that H. perforatum is not of hybrid origin, and for the first time wild diploid populations are documented. Pseudogamous facultative apomictic reproduction is prevalent in the polyploids, whereas diploids are predominantly sexual, a phenomenon which also characterizes its sister species H. maculatum. Both molecular markers characterize identical major gene pools, distinguishing H. perforatum from H. maculatum and two genetic groups in H. perforatum. All three gene pools are in close geographical contact. Extensive gene flow and hybridization throughout Europe within and between gene pools and species is exemplified by the molecular data and confirmed by morphometric analyses.

Conclusions Hypericum perforatum

is of a single evolutionary origin and later split into two major gene pools. Subsequently, independent and recurrent polyploidization occurred in all lineages and was accompanied by substantial gene flow within and between H. perforatum and H. maculatum. These processes are highly influenced by the reproductive system in both species, with a switch to predominantly apomictic reproduction in polyploids, irrespective of their origin.     

The effect of ploidy level on fitness in parthenogenetic flatworms

Although polyploidy plays an important role in speciation, its impact on fitness is still debated. One problem is that its adaptive significance can only be inferred by comparing forms with different ploidy that are identical in all other traits. This situation is uncommon, presumably because ploidy types often differ in reproduction mode, genetic background or habitat. Here we compare fitness in a system of triploid and tetraploid karyotypes of the planarian flatworm Schmidtea polychroa . Both types have the same type of sperm-dependent parthenogenesis and share the same genetic background and habitat. Hence, fitness differences, if any, can be attributed to different ploidy levels only. Contrary to the general assumption of a positive correlation between fitness and ploidy level, we showed that triploids produced 58% more offspring than tetraploids. Within each ploidy type, we identified groups of highly related clones using microsatellites. Significant variation among clonal groups in body size, offspring and cocoon number and hatching time indicated a genetic basis for variance in these traits. A small model shows that despite low fitness of tetraploids, stable coexistence of triploids and tetraploids can be explained by the recurrent origin of triploids from tetraploids and vice versa.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 191–198.     

Interspecific hybridization in natural populations ofCyrtandra (Gesneriaceae) on the Hawaiian Islands: Evidence from RAPD markers

Interspecific hybridization among Hawaiian species ofCyrtandra (Gesneriaceae) was investigated using randomly amplified polymorphic DNA (RAPD) markers. Thirty-three different primers were used to investigate interspecific hybridization for 17 different putative hybrids based on morphological intermediacy and sympatry with putative parental species. RAPD data provided evidence for the hybrid origin of all putative hybrid taxa examined in this analysis. However, the patterns in the hybrid taxa were not found to be completely additive of the patterns found in the parental species. Markers missing in the hybrid taxa can be attributed to polymorphism in the populations of the parental species and the dominant nature of inheritance for RAPD markers. Unique markers found within hybrid taxa require further explanation but do not necessarily indicate that the taxa are not of hybrid origin. The implications suggest that these interspecific hybridization events had, and continue to have, an effect on the adaptive radiation and conservation biology ofCyrtandra.