Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

The co-occurrence of apomixis (asexual reproduction) and polyploidy in plants has been the subject of debate in regard to the origin and evolution of asexuality. In recent years, polyploidy has been postulated as a maintenance and stabilization factor rather than as a source of apomixis origin. It is assumed polyploidy facilitates the compensation for mutation accumulation, and hence, the rare occurrence of diploid apomixis indirectly supports this finding. Nevertheless, diploid apomicts exist and are successful, especially in the genus Boechera. While comparing phenotypic traits, fitness-related traits and apomixis penetrance between both diploid and triploid apomicts in the genus Boechera, it was expected to find trait variance that can be attributed to ploidy. Surprisingly, little trait variation could be assigned to ploidy, but rather trait variations were mainly genotype-specific. Additionally, it is shown that paternal contribution is very important for trait success, even though all offspring are genetically identical to the mother plant. This harbors implications for the introduction of apomixis into crop plants, considering the effects of paternal contribution during asexual reproduction. Nevertheless, polyploidy is an efficient way to buffer deleterious mutations, but the flexibility of diploid apomicts of the genus Boechera for rare sexual events contributes to their success in nature.     

Quantitative variation for apomictic reproduction in the genus Boechera (Brassicaceae)

? Premise of the study: The evolution of asexual seed production (apomixis) from sexual relatives is a great enigma of plant biology. The genus Boechera is ideal for studying apomixis because of its close relation to Arabidopsis and the occurrence of sexual and apomictic species at low ploidy levels (diploid and triploid). Apomixis is characterized by three components: unreduced embryo-sac formation (apomeiosis), fertilization-independent embryogenesis (parthenogenesis), and functional endosperm formation (pseudogamy or autonomous endosperm formation). Understanding the variation in these traits within and between species has been hindered by the laborious histological analyses required to analyze large numbers of samples. ? Methods: To quantify variability for the different components of apomictic seed development, we developed a high-throughput flow cytometric seed screen technique to measure embryo:endosperm ploidy in over 22000 single seeds derived from 71 accessions of diploid and triploid Boechera. ? Key results: Three interrelated features were identified within and among Boechera species: (1) variation for most traits associated with apomictic seed formation, (2) three levels of apomeiosis expression (low, high, obligate), and (3) correlations between apomeiosis and parthenogenesis/pseudogamy. ? Conclusions: The data presented here provide a framework for choosing specific genotypes for correlations with large "omics" data sets being collected for Boechera to study population structure, gene flow, and evolution of specific traits. We hypothesize that low levels of apomeiosis represent an ancestral condition of Boechera, whereas high apomeiosis levels may have been induced by global gene regulatory changes associated with hybridization.     

Intraspecific diversification in North American Boechera stricta (= Arabis drummondii), Boechera xdivaricarpa, and Boechera holboellii (Brassicaceae) inferred from nuclear and chloroplast molecular markers--an integrative approach

We performed a combined evolutionary analysis of North American Boechera stricta, Boechera holboellii, and their hybrid Boechera ×divaricarpa using information on ploidy level estimators, allelic microsatellite variation, noncoding regions of the plastidic genome (cpDNA), and sequences of the internal transcribed spacers 1 and 2 of the nuclear ribosomal DNA (ITS). Somatic ploidy levels of herbarium specimens were estimated based on comparison of pollen size and the number of alleles per locus at seven microsatellites. Results indicate that B. stricta and B. holboellii are genetically distinct from each other, although we also find evidence for occasional introgression between both parental species. Microsatellite patterns for B. stricta from northeastern North America are genetically distinct from western populations, suggesting isolation in glacial refugia along the southeastern margin of the continuous ice shield. Microsatellites supported recent origin of B. ×divaricarpa. Correspondence of nrDNA with cpDNA genetic variation for the majority of diploid B. holboellii accessions suggests a basal, sexual evolutionary unit within a polymorphic B. holboellii group. Hybridization of genetically distinct lineage(s) evidently played an important role in the establishment of polyploid B. holboellii. Frequency of polyploid B. holboellii is substantially higher in the southern United States. This trend corresponds to a southerly distribution of derived chloroplast haplotypes, suggesting an evolutionary advantage of polyploidy and associated apomixis in the colonization of the Sierra Nevada and the Southern Rocky Mountains.     

The potential for rust infection to cause natural selection in apomictic Arabis holboellii (Brassicaceae)

Few studies have examined the potential for pathogens with complex life cycles to cause selection on their required alternate (=intermediate) hosts. Here we examine the effects of two fungal pathogens on an herbaceous mustard, Arabis holboellii. One pathogen species uses A. holboellii as a primary host, the other uses it as an alternate host. This plant-pathogen system is especially interesting because the host, A. holboellii, is apomictic; thus individuals reproduce exact copies of themselves. Despite this mode of reproduction, A. holboellii populations are surprisingly genetically diverse. Could frequency dependent selection by pathogens be maintaining clonal diversity? This study assesses the potential for selection by pathogens. In a controlled greehouse experiment we show that there is heritable variation in A. holboellii's resistance to the rust, Puccinia monoica, and that host fitness is severely reduced by P. monoica infection in both the greenhouse and under natural conditions. Field observations indicate that host clones are also differentially susceptible to the short-cycled rust, P. thlaspeos, and that host fitness is reduced by infection to this pathogen as well. Although the preconditions for pathogen-mediated selection are present, frequency-dependent selection by pathogens is unlikely to be important in structuring populations of Arabis holboellii because multiple host genotypes are susceptible to the same inoculum and the pathogen has a long generation time.     

The distribution of B chromosomes across species

In this review we look at the broad picture of how B chromosomes are distributed across a wide range of species. We review recent studies of the factors associated with the presence of Bs across species, and provide new analyses with updated data and additional variables. The major obstacle facing comparative studies of B chromosome distribution is variation among species in the intensity of cytogenetic study. Because Bs are, by definition, not present in all individuals of a species, they may often be overlooked in species that are rarely studied. We give examples of corrections for differences in study effort, and show that after a variety of such corrections, strong correlations remain. Several major biological factors are associated with the presence of B chromosomes. Among flowering plants, Bs are more likely to occur in outcrossing than in inbred species, and their presence is also positively correlated with genome size and negatively with chromosome number. They are no more frequent in polyploids than in diploids, nor in species with multiple ploidies. Among mammals, Bs are more likely to occur in species with karyotypes consisting of mostly acrocentric chromosomes. We find no evidence for an association with chromosome number or genome size in mammals, although the sample for genome size is small. The associations with breeding system and acrocentric chromosomes were both predicted in advance, but those with genome size and chromosome number were discovered empirically and we can offer only tentative explanations for the very strong associations we have uncovered. Our understanding of why B chromosomes are present in some species and absent in others is still in its infancy, and we suggest several potential avenues for future research.     

Molecular biogeography of clonal lineages in a high-Arctic apomictic Daphnia complex

An electrophoretic survey of 81 populations of arctic Daphnia pulex from around the Svalbard archipelago revealed the presence of 49 unique allozyme clones ( N = 3357). Two closely related clones accounted for 66% of the total sample, and were widespread across the archipelago. Restriction fragment length polymorphisms (RFLPs) of a 2.1-kb fragment of mtDNA (NADH-4 and NADH-5 subunits), amplified using the polymerase chain reaction (PCR), revealed the presence of eight mtDNA haplotypes. One haplotype was particularly widespread, and the two most abundant allozyme clones shared this haplotype. Nonrandom distribution patterns of clones were observed, and are most likely the result of historical events (i.e. founder effects) related to the past glacial history of the archipelago. The data are discussed with reference to past glaciation events, and attempts are made to discern the colonization history of this apomictic complex.     

On the origin and evolution of apomixis in Boechera

The genetic mechanisms causing seed development by gametophytic apomixis in plants are predominantly unknown. As apomixis is consistently associated with hybridity and polyploidy, these confounding factors may either (a) be the underlying mechanism for the expression of apomixis, or (b) obscure the genetic factors which cause apomixis. To distinguish between these hypotheses, we analyzed the population genetic patterns of diploid and triploid apomictic lineages and their sexual progenitors in the genus Boechera (Brassicaceae). We find that while triploid apomixis is associated with hybridization, the majority of diploid apomictic lineages are likely the product of intra-specific crosses. We then show that these diploid apomicts are more likely to sire triploid apomictic lineages than conspecific sexuals. Combined with flow cytometric seed screen phenotyping for male and female components of apomixis, our analyses demonstrate that hybridization is an indirect correlate of apomixis in Boechera.     

Deciphering the origins of apomictic polyploids in the Cheilanthes yavapensis complex (Pteridaceae)

Deciphering species relationships and hybrid origins in polyploid agamic species complexes is notoriously difficult. In this study of cheilanthoid ferns, we demonstrate increased resolving power for clarifying the origins of polyploid lineages by integrating evidence from a diverse selection of biosystematic methods. The prevalence of polyploidy, hybridization, and apomixis in ferns suggests that these processes play a significant role in their evolution and diversification. Using a combination of systematic approaches, we investigated the origins of apomictic polyploids belonging to the Cheilanthes yavapensis complex. Spore studies allowed us to assess ploidy levels; plastid and nuclear DNA sequencing revealed evolutionary relationships and confirmed the putative progenitors (both maternal and paternal) of taxa of hybrid origin; enzyme electrophoretic evidence provided information on genome dosage in allopolyploids. We find here that the widespread apomictic triploid, Cheilanthes lindheimeri, is an autopolyploid derived from a rare, previously undetected sexual diploid. The apomictic triploid Cheilanthes wootonii is shown to be an interspecific hybrid between C. fendleri and C. lindheimeri, whereas the apomictic tetraploid C. yavapensis is comprised of two cryptic and geographically distinct lineages. We show that earlier morphology-based hypotheses of species relationships, while not altogether incorrect, only partially explain the complicated evolutionary history of these ferns.     

Isozyme and morphological diversity within apomictic and sexual taxa of the Ranunculus auricomus complex

 In 17 populations of the tetraploid agamospecies R. variabilis, 10 enzyme systems have been studied by horizontal starch gel electrophoresis, and compared to previous data of 3 populations of the diploid sexual species R. notabilis. Allozymic and genotypic diversity of 12 polymorphic loci, and multilocus genotype diversity in R. variabilis compares to other apomictic taxa, confirming the assumed apomictic mode of reproduction previously found in 3 populations. A cluster analysis based on genetic distance values separated the two taxa, the populations of southeastern Austria sympatric with R. notabilis showing no closer relationships to the sexual species than the allopatric ones. The R. variabilis populations of the circumalpine regions are separated as a rather uniform group, those of the Bohemian massif as another, more heterogeneous group. This geographic differentiation is mainly due to genotypic variation, which is higher in the Bohemian massif than in the circumalpine group, and lower in areas where R. variabilis is sympatric with R. notabilis. Proportion of multilocus genotypes within populations (G/N), genotype diversity (D), and genotype evenness (E) measures indicate facultative recombination events with fixation and clonal reproduction of new recombined genotypes, which is regarded as the most important factor for the evolution of new lineages in goldilocks. Morphometric data of the R. variabilis and R. notabilis populations studied by cluster and principal coordinate analyses clearly separated the two taxa, and indicated a slight geographic differentiation with in R. variabilis corresponding to the results of isozyme data. A principal coordinate analysis of R. variabilis individuals based on stem and fruit characters showed a clinal variation, but no clear separation of groups. The variation found in this PCoA is almost completely covered from the most widespread clone (genotype I), and also from individual genotypes. The implications of the results on different species concepts for apomicts are discussed. For R. variabilis, no taxonomic conclusions can be drawn from the present data set. Received December 15, 1999 Accepted November 15, 2000     

Interaction of B chromosomes with A or B chromosomes in segregation in insects

Additional or B chromosomes not belonging to the regular karyotype of a species are found in many animal and plant groups. They form a highly heterogeneous group with respect to their morphology and behaviour both in mitosis and meiosis. Achiasmatic mechanisms that ensure the segregation of a B chromosome from another B chromosome or from an A chromosome are reviewed. An achiasmatic mechanism characterized by the "distance pairing" of segregating univalents at metaphase I was found to be responsible for the preferential segregation of B chromosome univalents in Hemerobius marginatus L. (Neuroptera), and a mechanism characterized by the "touch and go pairing" of segregating univalents was responsible for the highly regular segregation of a B chromosome and the X chromosome in Rhinocola aceris (L.) (Psylloidea, Homoptera). The latter mechanism resulted in the integration of a B chromosome to the A chromosome set as a Y chromosome in a psyllid species Cacopsylla peregrina (Frst.). Furthermore, B chromosomes can disturb the regular segregation of the achiasmatic X and Y chromosomes resulting in the formation of X0/XY polymorphism in a population, which might precede the loss of the Y chromosome. The absence of observations on accurately functioning achiasmatic segregation mechanisms in grasshoppers (Orthoptera) was attributed to the X and B chromosomes, which re-orient one or several times during metaphase I. Apparently, these re-orientations mask any achiasmatic segregation mechanism that might operate during meiotic prophase in these insects.     

Biogeographic and demographic changes in the distribution and abundance of scorpions inhabiting the Mediterranean region in northern Israel

A recent survey of the scorpion fauna in the Mediterranean region of northern Israel, has shown that the desert buthid, Leiurus quinquestriatus, formerly restricted to the arid and semi-arid regions in southern and eastern Israel, has penetrated through the Jordan Valley deep into the Mediterranean region, reaching into the northern Galil Mountains. At the same time, the oakwood scorpionid, Scorpio maurius fuscus, formerly the most abundant scorpion in the Mediterranean region, showed a marked decline in numbers. Various aspects of this apparent colonization and replacement in the scorpion fauna are discussed.     

Multiple chromosomes in Burkholderia cepacia and B. gladioli and their distribution in clinical and environmental strains of B. cepacia

Burkholderia cepacia is found in soils and waters, it can be used in biocontrol and bioremediation but is also a human pathogen. It is not yet clear what differentiates pathogenic from non-pathogenic strains of the organism. In this study the multiple replicon structure was investigated in 28 strains of B. cepacia by pulsed field gel electrophoresis. All strains examined, whether of clinical, environmental or plant pathogenic origin, were found to have two, three or four large (> 500 kbp) replicons. Many strains also contained small replicons. Clinical strains were more likely to have three or four large replicons than non-clinical strains. Multiple replicon structure was also demonstrated in B. gladioli and Alcaligenes eutrophus.     

Variation in the distribution of a genome-specific DNA sequence on chromosomes reveals evolutionary relationships in the Triticum and Aegilops complex

 The present study analyzed the distribution pattern of the Ae. speltoides–derived repetitive clone pGc1R-1 in the Triticum/Aegilops complex. Fluorescence in situ hybridization analysis showed that clone pGc1R-1 is a S-genome-specific repetitive sequence that hybridized to the S-genome of three species in the section Sitopsis, Aegilops speltoides (S), Ae. longissima (S^l), and Ae. sharonensis (S^sh), but not to Ae. bicornis (S^b) and Ae. searsii (S^s), nor to any other diploid Aegilops species. This clone also hybridized to the very closely related G-genome of T. timopheevii subsp. armeniacum and T. timopheevii ssp. timopheevii, but not to the B-genome of T. turgidum and T. aestivum. Hybridization also was observed in the polyploid Aegilops species, Ae. kotschyi (U^kS^k), Ae. peregrina (U^pS^p), and Ae. vavilovii (X^vaD^vaS^va). Large inter- and intraspecific variations were observed. Our results confirm that the S genome is related more to the S^l and S^sh genomes than to the S^b and S^s genomes; there is a greater affinity between the G and S genomes than between the B and S genomes. Mechanisms to account for the variation in the FISH pattern with different genomes include sequence amplification and deletion. Variation in the distribution of this genome-specific DNA sequence, pGc1R-1, on chromosomes can be used to reveal evolutionary relationships in the Triticum and Aegilops complex. Received April 10, 2002; accepted July 12, 2002 Published online: November 28, 2002 Address of the authors: Peng Zhang, Bernd Friebe (e-mail: friebe@ksu.edu), Bikram S. Gill, Wheat Genetics Resource Center, Department of Plant Pathology, 4024 Throckmorton, Plant Sciences Center, Kansas State University, Manhattan, KS 66506-5502, USA.     

Differential distribution of RNA polymerase B and nonhistone chromosomal proteins in polytene chromosomes of Drosophila melanogaster

The distribution patterns of nonhistone chromosomal proteins (NHCP) associated with pulse-labeled RNA were determined by indirect immunofluorescence on salivary gland chromosomes of Drosophila melanogaster using monoclonal antibodies. By staining for two different antigens simultaneously, using antibodies tagged with different fluorescent probes, it became possible to position RNA-associated antigens as well as RNA polymerase B in relation to each other. Three separate staining patterns could be observed with anti-NHCP antibodies, none of which showed a pattern which was identical with that of RNA polymerase B. Furthermore, no correlation with the synthesis of the primary trancript, as monitored by the RNA polymerase B content of chromosomal sites, could be found by following the fluorescence patterns during inactivation of intermolt puffs or activation of early ecdysone-induced puffs. Finally, no strict correlation was observed between puffing activity and the accumulation of a certain antigen in these selected chromosomal sites.     

B chromosomes in Brazilian rodents

B chromosomes are now known in eight Brazilian rodent species: Akodon montensis, Holochilus brasiliensis, Nectomys rattus, N. squamipes, Oligoryzomys flavescens, Oryzomys angouya, Proechimys sp. 2 and Trinomys iheringi. Typically these chromosomes are heterogeneous relative to size, morphology, banding patterns, presence/absence of NORs, and presence/absence of interstitial telomeric signals after FISH. In most cases, Bs are heterochromatic and late replicating. Active NORs were detected in two species: Akodon montensis and Oryzomys angouya. As a rule, Bs behave as uni or bivalents in meiosis, there is no pairing between Bs and autosomes or sex chromosomes and also their synaptonemal complexes are isopycnotic with those in A chromosomes.     

The B chromosomes in Brachycome

This review presents a historical account of studies of B chromosomes in the genus Brachycome Cass. (synonym: Brachyscome) from the earliest cytological investigations carried out in the late 1960s though to the most recent molecular analyses. Molecular analyses provide insights into the origin and evolution of the B chromosomes (Bs) of Brachycome dichromosomatica, a species which has Bs of two different sizes. The larger Bs are somatically stable whereas the smaller, or micro, Bs are somatically unstable. Both B types contain clusters of ribosomal RNA genes that have been shown unequivocally to be inactive in the case of the larger Bs. The large Bs carry a family of tandem repeat sequences (Bd49) that are located mainly at the centromere. Multiple copies of sequences related to this repeat are present on the A chromosomes (As) of related species, whereas only a few copies exist in the A chromosomes of B. dichromosomatica. The micro Bs share DNA sequences with the As and the larger Bs, and they also have B-specific repeats (Bdm29 and Bdm54). In some cases repeat sequences on the micro Bs have been shown to occur as clusters on the A chromosomes in a proportion of individuals within a population. It is clear that none of these B types originated by simple excision of segments from the A chromosomes.     

The role of hybridization, polyploidization and glaciation in the origin and evolution of the apomictic Ranunculus cassubicus complex

The Ranunculus cassubicus complex, comprising diploids and polyploids, is a good model for studying the role of hybridization and polyploidy in the origin of apomixis. Results from amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) analyses performed on 448 individuals were combined with evidence from morphology, isozymes, karyology and distribution. Our results indicated a unique hybrid origin for the apomictic hexaploid R. carpaticola from north-western Slovakia, involving two sexual parents: autotetraploid R. cassubicifolius from the northern pre-Alps, and diploid R. carpaticola from central Slovakia. The hybrids were intermediate to the parents, but unique alleles have resulted from genomic reorganisation in the allopolyploids, which might also have triggered apomixis. Their distribution patterns and estimated ages suggest that hybridization may be correlated with the last glacial period. Hybridization seems to be the major origination for apomicts in the R. cassubicus complex. Polyploidy creates novel sexual genotypes and acts as a springboard for the production of new hybrids, but it only results in a combination with hybridization in apomixis. In turn, asexuality has permitted the perpetuation and establishment of ecologically divergent hybrid genotypes.