Evolution by Reticulation: European Dogroses Originated by Multiple Hybridization Across the Genus Rosa
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The European dogroses (Rosa sect. Caninae (DC.) Ser.) are characterized by a unique meiosis system ("canina-meiosis"), which controls the heterogamous development of tetraploid egg cells and haploid pollen grains resulting in a pentaploid somatic status. This permanent anorthoploidy is supposed to have originated by a hybridization event in the postglacial period. In this study we present molecular evidence by an analysis of nuclear ribosomal DNA data that dogroses are complex allopolyploids resulting from multiple hybridization events. As previously described, the nrITS-1 region does not undergo concerted evolution in dogroses. Thus, different ITS-1 sequences persist within single individuals. Secondary structure predictions do not point to the existence of pseudogenes within these ITS-1 types. Our data suggest that the pentaploid Caninae genome originated from different members of nondogroses and the now extinct Protocaninae.Keywords:
Pseudogene
Concerted evolution
Pseudogene
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Pseudogene
genomic DNA
Concerted evolution
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Abstract The fungal sexual cycle is, in its alternation of haploid and diploid phases, essentially the same as in higher plants and animals, with fusion of haploid gamete nuclei (karyogamy) to give diploidy, and meiosis to restore haploidy. Where most fungi differ from ‘higher’ eukaryotes is that, meiosis follows immediately after karyogamy, so that the diploid phase is confined to the meiotic cell. In the most studied groups of fungi, Ascomycetes and Basidiomycetes, which are sexually reproducing, there are differences in their modes of formation of haploid spores.
Gamete
Sexual reproduction
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The nuclear ribosomal DNA (rDNA) is considered as a paradigm of concerted evolution. Components of the rDNA tandem repeats (45S) are widely used in phylogenetic studies of different organisms and the internal transcribed spacer (ITS) region was recently selected as a fungal DNA bar code. However, rRNA pseudogenes, as one kind of escape from concerted evolution, were reported in a wide range of organisms, especially in plants and animals. Moreover, large numbers of 5S rRNA pseudogenes were identified in several filamentous ascomycetes. To study whether rDNA evolves in a strict concerted manner and test whether rRNA pseudogenes exist in more species of ascomycetes, intragenomic rDNA polymorphisms were analyzed using whole genome sequences. Divergent rDNA paralogs were found to coexist within a single genome in seven filamentous ascomycetes examined. A great number of paralogs were identified as pseudogenes according to the mutation and secondary structure analyses. Phylogenetic analyses of the three rRNA coding regions of the 45S rDNA repeats, i.e., 18S, 5.8S, and 28S, revealed an interspecies clustering pattern of those different rDNA paralogs. The identified rRNA pseudogenic sequences were validated using specific primers designed. Mutation analyses revealed that the repeat-induced point (RIP) mutation was probably responsible for the formation of those rRNA pseudogenes.
Pseudogene
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Paspalum equitans is a sexual, cross-pollinated diploid (2n = 2x = 20) with normal meiosis. It has some potential for apomixis because one or two aposporous embryo sacs were observed beside the meiotic sac in 18.2% of the ovules. Regular meiotic behavior was confirmed in tetraploid (2n = 4x = 40) P. ionanthum (= P. guaraniticum), which has 20II at meiosis and is cross-pollinated. Hybrids were produced between P. equitans and diploid P. cromyorrhizon, a species with the basic N genome. A high degree of meiotic chromosome pairing in the hybrids demonstrated that P. equitans also has the N genome. However, these species are isolated because the F1 hybrids are sterile. Meiotic chromosome pairing in hybrids of P. ionanthum x diploid and tetraploid P. cromyorrhizon indicated that at least one genome of P. ionanthum is a form of the N genome. Moreover, 1-7III were observed in ca. 75% of pollen mother cells of P. ionanthum x diploid P. cromyorrhizon triploid hybrids, and quadrivalents were observed in P. ionanthum x tetraploid P. cromyorrhizon hybrids, which indicated that P. ionanthum contributed two homoeologous genomes. In the diploid P. cromyorrhizon x P. ionanthum cross, a tetraploid hybrid was recovered, indicating that an unreduced diploid gamete of P. cromyorrhizon was involved in the origin of this hybrid.
Apomixis
Paspalum
Polyploid
Gamete
Chromosome pairing
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Development of megaspores and megagametophytes was analyzed for several diploid potato clones (Solanum spp.) that exhibit either high (HI) or low (LO) seed set when crossed as female with the tetraploid cultivated potato S. tuberosum Group Tuberosa. The objectives were to determine the relationship between ploidy and diam of nuclei and nucleoli, and to determine the mechanism(s) and frequencies of 2n megagametophyte formation. Sizes of nuclei and nucleoli were found to depend on ploidy. For HI clones, the distributions of sizes indicated that doubling occurred during meiosis, and that 30 to 50% of the megaspores and megagametophytes were 2n rather than haploid. Omission of the second meiotic division led to formation of second division restitution (SDR) 2n megagametophytes. Only one HI clone had abnormal meiosis I, in addition to omission of meiosis II in some meiocytes; this clone seemed to produce not only 1n and 2n, but also 4n megagametophytes. The results indicated that high crossability of the HI clones as female with tetraploids largely was due to formation of SDR 2n megagametophytes, a finding strongly supporting the hypothesis that sexual polyploidization is the driving force behind polyploidization of Solanums. The results contribute to increasing evidence that meiotic mutants and abnormalities play an important role in angiosperm evolution.
Megaspore
Meiocyte
clone (Java method)
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Meiosis and pollen mitosis were studied in a clone of Solanum phureja in order to elucidate the generally high frequency (about 90 %) of tetraploid progeny in tetraploid X diploid crosses involving S. phureja. The first meiotic division was normal with 12 bivalents in metaphase and apparently normal chromosome disjunction. The second meiotic division showed normal metaphases with 12 chromosomes in each of the two spindles, along with cells showing a bi- or tripolar spindle (cell wall formation was simultaneous). At the end of the second division those disturbances resulted in unreduced cells either with two diploid nuclei or with one diploid and two haploid nuclei along with the normal tetrads with four haploid nuclei. The frequency of unreduced gametes was about 50 %. First and second pollen mitoses were also represented by normal haploid as well as unreduced diploid cells. Hence, the high frequency of tetraploid offspring in Solanum tuberosum X S. phureja crosses could be caused by selective fertilization with unreduced diploid pollen grains.
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SummaryThe botanical, biological characters, male meiosis and offspring chromosome numbers of a haploid Capsicum annuum L. were observed. There were some differences in botanical and biological characters between the haploid and the diploid. In the haploid, natural chromosome doubling occurred due to unreduced eggs, 0.127 seeds per fruit developed, and all five offspring seedlings were restored to diploidy. At meiotic pachytene, a pseudosynaptic structure occurred. During male meiosis of the haploid, 14.2% dyads were observed, which was through the ®rst division restitution mechanism, and 9.8% unreduced viable pollen was collected. The results would have considerable importance in the study of the occurrence and cytogenetic mechanism of 2n gametes; in the evaluation of the effects of haploidy on plant evolution or breeding, and in studies on chromosome recognition and pairing.
Capsicum annuum
Chromosome number
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Chromosome number and meiotic behavior were evaluated in 36 Brazilian accessions of the grass Paspalum (which had never previously been analyzed) to determinate which accessions might be useful in interspecific hybridizations. The analysis showed that one accession of Paspalum coryphaeum was diploid (2n = 2x = 20) and one accession of Paspalum conspersum hexaploid (2n = 6x = 60), the remaining 34 accessions being tetraploid (2n = 4x = 40). The pairing configuration was typical for the ploidy level i.e. in the diploid, chromosomes paired as 10 bivalents, in tetraploids as bi-, tri- and quadrivalents, and in hexaploid as 30 bivalents. A low frequency of meiotic abnormalities (less than 10%) was observed in the diploid, hexaploid and some tetraploid accessions, although the majority of tetraploid accessions showed a high frequency of meiotic irregularities. The use of accessions with a low frequency of meiotic abnormalities in breeding programs is discussed.
Paspalum
Chromosome pairing
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Development of megaspores and megagametophytes was analyzed for several diploid potato clones ( Solanum spp.) that exhibit either high (HI) or low (LO) seed set when crossed as female with the tetraploid cultivated potato S. tuberosum Group Tuberosa. The objectives were to determine the relationship between ploidy and diam of nuclei and nucleoli, and to determine the mechanism(s) and frequencies of 2 n megagametophyte formation. Sizes of nuclei and nucleoli were found to depend on ploidy. For HI clones, the distributions of sizes indicated that doubling occurred during meiosis, and that 30 to 50% of the megaspores and megagametophytes were 2 n rather than haploid. Omission of the second meiotic division led to formation of second division restitution (SDR) 2 n megagametophytes. Only one HI clone had abnormal meiosis I, in addition to omission of meiosis II in some meiocytes; this clone seemed to produce not only 1 n and 2 n , but also 4 n megagametophytes. The results indicated that high crossability of the HI clones as female with tetraploids largely was due to formation of SDR 2 n megagametophytes, a finding strongly supporting the hypothesis that sexual polyploidization is the driving force behind polyploidization of Solanums. The results contribute to increasing evidence that meiotic mutants and abnormalities play an important role in angiosperm evolution.
Megaspore
Meiocyte
clone (Java method)
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Citations (31)