The significance of genic balance to endosperm development in interspecific crosses

Summary The endosperm has played a significant role in the evolution of angiosperms because of its physiological and genetic relationships to the embryo. One manifestation of this evolutionary role is its abnormal development in interploidy crosses. It is now established that the endosperm develops abnormally in interploidy-intraspecific crosses when the maternal: paternal genome ratio deviates from 21 in the endosperm itself. We propose an Endosperm Balance Number (EBN) hypothesis to explain endosperm development in both interploidy-intraspecific and interspecific crosses. Each species is assigned an EBN on the basis of its crossing behavior to a standard species. It is the EBN which determines the effective ploidy in the endosperm of each species, and it is the EBNs which must be in a 21, maternal:paternal ratio. The EBN of a species may be determined by a few genes rather than the whole genome. This hypothesis brings most intraspecific-interploidy and interspecific crossing data under a single concept with respect to endosperm function. The implications of this hypothesis to isolating mechanisms, 2n gametes, the evolution of disomic polyploids, and reciprocal differences in seed development are discussed.     

Polyploidy‐associated genomic instability in Arabidopsis thaliana

Formation of polyploid organisms by fertilization of unreduced gametes in meiotic mutants is believed to be a common phenomenon in species evolution. However, not well understood is how species in nature generally exist as haploid and diploid organisms in a long evolutionary time while polyploidization must have repeatedly occurred via meiotic mutations. Here, we show that the ploidy increased for two consecutive generations due to unreduced but viable gametes in the Arabidopsis cyclin a1;2‐2 (also named tardy asynchronous meiosis‐2) mutant, but the resultant octaploid plants produced progeny of either the same or reduced ploidy via genomic reductions during meiosis and pollen mitosis. Ploidy reductions through sexual reproduction were also observed in independently generated artificial octaploid and hexaploid Arabidopsis plants. These results demonstrate that octaploid is likely the maximal ploidy produced through sexual reproduction in Arabidopsis. The polyploidy‐associated genomic instability may be a general phenomenon that constrains ploidy levels in species evolution. genesis 48:254–263, 2010. © 2010 Wiley‐Liss, Inc.     

Pathways to polyploidy: indications of a female triploid bridge in the alpine species <Emphasis Type="Italic">Ranunculus kuepferi</Emphasis> (Ranunculaceae)

Polyploidy is one of the most important evolutionary processes in plants. In natural populations, polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. The frequencies of these two pathways, and male versus female gamete contributions, however, are largely unexplored. Ranunculus kuepferi occurs with diploid, triploid and autotetraploid cytotypes in the Alps, whereby diploids are mostly sexual, while tetraploids are facultative apomicts. To test for the occurrence of polyploidization events by triploid bridge, we investigated 551 plants of natural populations via flow cytometric seed screening. We assessed ploidy shifts in the embryo to reconstruct female versus male gamete contributions to polyploid embryo and/or endosperm formation. Seed formation via unreduced egg cells (B_III hybrids) occurred in all three cytotypes, while only in one case both gametes were unreduced. Polyploids further formed seeds with reduced, unfertilized egg cells (polyhaploids and aneuploids). Pollen was highly variable in diameter, but only pollen >27 μm was viable, whereby diploids produced higher proportions of well-developed pollen. Pollen size was not informative for the formation of unreduced pollen. These results suggest that a female triploid bridge via unreduced egg cells is the major pathway toward polyploidization in R. kuepferi, maybe as a consequence of constraints of endosperm development. Triploids resulting from unreduced male gametes were not observed, which explains the lack of obligate sexual tetraploid individuals and populations. Unreduced egg cell formation in diploids represents the first step toward apomixis.     

Post-zygotic gametic selection due to endosperm balance number explains unusual chromosome numbers of 3×× 2× progeny in Solanum

 Crosses between triploid and diploid genotypes are usually the best sources of trisomics in potato as well as in several other crop species. However, 3×× 2× crosses between triploid (2n=3×=36; 2EBN) Solanum commersonii-S. tuberosum hybrids and diploid (2n= 2×=24; 2EBN) genotypes gave progenies with a high number of extra chromosomes, 29–36, suggesting that only eggs with 17–24 chromosomes produced embryos that reached full development. Our hypothesis is that although triploids produce eggs with a range of chromosome numbers, 3×× 2× crosses involving a 2×(2EBN) parent favor eggs with a high chromosome number. These eggs have higher probabilities of possessing the same endosperm balance number (EBN) value (i.e. 1) of gametes produced by the 2EBN diploid parent to give the required 2:1 maternal to paternal EBN ratio in the hybrid endosperm. Under this model, trisomics are produced only if the diploid parent has an EBN of 1. Based on our results and those reported in the literature, it is proposed that in 3×(2EBN) × 2×(2EBN) crosses the endosperm balance number exercises negative selection for gametes with a low chromosome number, and a corresponding low EBN, and positive selection for gametes with a high chromosome number and EBN. Received: 2 April 1998 / Revision accepted: 27 October 1998     

植物多倍体基因组的形成与进化

多倍化是植物进化变异的自然现象,也是促进植物发生进化改变的重要力量。在被子植物中,约 70％的种类在进化史中曾发生过一次或多次多倍化的过程。目前的研究结果表明,自然界绝大多数多倍体是通过未减数配子的融合而形成的,并且很多多倍体种是通过多次独立的多倍化过程而重复发生的。由多倍化所导致的重复基因在多倍体基因组中可能有三种不同的命运,即：保持原有的功能、基因沉默或分化并执行新的功能。多倍化以后,重复基因组的进化动态则主要表现在染色体重排和“染色体二倍化”、不同基因组之间的相互渗透、以及核-质之间的相互作用等方面。     

Association of ploidy and sexual system in Lycium californicum (Solanaceae)

In North American Lycium (Solanaceae), the evolution of gender dimorphism has been proposed as a means of restoring outcrossing after polyploidization causes the loss of self-incompatibility. Previous studies of this process in Lycium focused on comparisons between species that differ in ploidy. We examined intraspecific variation in floral morphology and DNA content in populations of L. californicum to determine correlations between sexual system and cytotype. We also used nuclear ITS and GBSSI sequence data to determine whether diploid and polyploid forms represent the same phylogenetic species, and the phylogeographic relationships among populations and ploidy levels. Within populations, no variation in ploidy was found, although among populations there was a perfect correspondence between sexual system and cytotype. Diploid populations were all hermaphroditic, whereas tetraploid populations were all gender dimorphic. There was no clear geographic pattern to the occurrence of diploid and tetraploid forms. Phylogenetic analysis confirms that L. californicum, regardless of ploidy, forms a monophyletic group within the genus Lycium. Sequences from diploid and polyploid individuals did not form reciprocally monophyletic clades, indicating either multiple gains of polyploidy, ongoing gene flow between cytotypes, or lack of lineage sorting since the evolution of polyploidy. The correspondence between ploidy and sex expression is consistent with the hypothesis that polyploidization triggers the evolution of gender dimorphism in this and other Lycium species.     

Chromosome numbers, nuclear DNA content, and polyploidy in Consolea (Cactaceae), an endemic cactus of the Caribbean Islands

Polyploidy, an important mechanism of plant evolution, was investigated in Consolea, an endemic Caribbean opuntioid genus represented by nine subdioecious species with very narrow distributions, including species classified as rare or threatened. Standard chromosome counting and flow cytometric analyses were used to determine chromosome numbers and ploidy of each taxon. Compared to the base number (x = 11), the mitotic and meiotic counts indicated that there are seven hexaploid (2n = 66) and two octoploid species (2n = 88); no diploids were found. Histograms of intact nuclei confirmed that all species are polyploid, with C-DNA values ranging from 4.88-9.50 pg. The variation of DNA content was significantly higher for the octoploids than for the hexaploids. Male and female sexual morphs had similar DNA content, suggesting that there are no sex chromosomes. Cytomixis between cells and microsporocytes with no chromatin were observed. This provides a mechanism whereby gametes with variable chromosome numbers are produced, influencing reproduction and promoting speciation. In conclusion, C-DNA content and chromosome number separated Consolea species into two groups, which may correspond to two phylogenetic lineages or indicate that polyploidization occurred independently, with comparable effects on C-DNA content.     

Ploidy level manipulations in potato through sexual hybridisation

There is no better use of sexual reproduction in regard to breeding and genetic research than the ploidy level manipulations possible in the potato and its relatives. Unique reproductive characteristics of tuber‐bearing Solanum species make possible: the production of gametes with unreduced chromosome number; the presence of an endosperm dosage system that regulates success of interploidy/interspecific crosses; the possibility to easily extract maternal haploids following crosses with S. phureja. This paper reviews results obtained in scaling genomic multiples up and down in potato, and relates these manipulations to breeding strategies for the genetic improvement of the cultivated potato. Several ploidy series have been developed, ranging from the monoploid to the hexaploid level. Cultivated tetraploids were scaled down to the diploid and monoploid level by haploidy. Scaling upward was achieved by sexual polyploidisation via 2n gametes that resulted in triploid, tetraploid, pentaploid, and hexaploid genotypes with a broad genetic base. Altogether, the success of ploidy level manipulations constitutes further proof that sexual polyploidisation played an important role in the polyploid evolution of Solanum species, and supports the idea that gene flow can be relatively easily accomplished through interploid and bridge crosses.     

The occurrence and frequency of 2n pollen in 2x, 4x,and 6x wild,tuber-bearing Solanum species from Mexico,and Central and South America

Summary The occurrence of 2n pollen-producing plants was investigated in 187 plant introductions (PIs) of 38 wild species of tuber-bearing Solanum. These 2x, 4x, and 6x species are from Mexico, and Central and South America. The determination of 2n pollen-producing plants was conducted using acetocarmine glycerol. Plants with more than 1% large-size pollen were regarded as 2n pollen-producing plants. 2n pollen-producing plants were identified in the following species: 10 out of 12 Mexican 2x species, seven of nine South American 2x species, seven of seven Mexican and Central American 4x species, five of five South American 4x species, and five of five Mexican 6x species. The frequency of 2n pollen-producing plants varied among species at the same ploidy level, but the range of frequency, generally between 2 and 10% among species, was similar over different ploidy levels. The general occurrence of 2n pollen in both 2x and polyploid species, which are evolutionarily related, is evidence that the mode of polyploidization in tuber-bearing Solanums is sexual polyploidization. Furthermore, the frequencies of 2n pollen-producing plants in autogamous disomic polyploid species were not markably different from those of their related diploid species. It is thought that the frequent occurrence of 2n gametes with autogamy tends to disturb the fertility and consequently reduce fitness of polyploids. Thus, we propose that the breeding behavior of polyploids and the occurrence of 2n gametes may be genetically balanced in order to conserve high fitness in polyploid species in tuberbearing Solanum.Paper No. 3114 from the Laboratory of Genetics. Research supported by the College of Agriculture and Life Sciences; International Potato Center; USDA, SEA, CGRO 84-CRCR-1-1389; and Frito Lay, Inc.     

Effect of pollen source and pollen ploidy on endosperm formation and seed set in pseudogamous apomictic Paspalum notatum

 It is generally accepted that most angiosperms require an accurate balance between maternal and paternal genome contribution for endosperm development. The endosperm balance number (EBN) hypothesis postulates that each species has an effective number which must be in a 2:1 maternal to paternal ratio for normal endosperm development and seed formation. The aim of this work was to investigate the effect of different sources and ploidy levels of pollen donors on endosperm formation and seed production of aposporous tetraploid (2n=4×=40) Paspalum notatum. Hand-emasculated spikelets of an apomictic 4× plant were dusted with pollen of 2×, 4×, 5×, 6× and 8× races of the same species; 3× and 4× races of a phylogenetically closely related species, P. cromyorrhizon; and 2× and 4× races of P. simplex, a species of a different subgenus. Experiments including self-pollination as well as emasculation without pollination were conducted for controls. Results indicated that apomictic 4×P. notatum is a pseudogamous species with effective fertilization of the two unreduced (2n) polar nuclei by a reduced (n) sperm. Endosperm development and seed production occurred independently of the species or the ploidy level of the pollen donor. However, seed germination rates were significantly lower than in the self-pollinated control when the pollen donor was 3×P. cromyorrhizon or 2× and 4×P. simplex. Aposporous embryo sacs in Paspalum contribute to endosperm formation with two unreduced (2n) polar nuclei, while the male contribution is the same as in sexual plants (n). Since sexual Paspalum plants fit the EBN hypothesis, the EBN insensitivity observed in apomictic plants might be a prerequisite for the spread of pseudogamous apomixis. The EBN insensitivity could have arisen as an imprinting consequence of a high maternal genome contribution. Received: 20 February 1998 / Revision accepted: 21 October 1998     

Substitutes for genome differentiation in tuber-bearing <Emphasis Type="Italic">Solanum</Emphasis>: interspecific pollen-pistil incompatibility,nuclear-cytoplasmic male sterility,and endosperm

The cultivated potato, Solanum tuberosum L. (2n=4x=48), has a very large number of related wild and cultivated tuber-bearing species widely distributed in the Americas. These species, grouped in 16 taxonomic series, range from the diploid to the hexaploid level. Polyploid species are either disomic or polysomic, and sexual polyploidization via genetically controlled 2n gametes has played a major role in their evolution. Species are separated in nature by geographical and ecological barriers. However, there are several examples of sympatric species that share the same niches but do not readily cross (i.e., the diploids S. commersonii and S. chacoense in certain areas of Argentina). External barriers alone are, therefore, not sufficient to explain species integrity. In addition, there is no strong evidence indicating that genome differentiation is important in the group. In this review we present evidence supporting the assertion that interspecific pollen-pistil incompatibility, nuclear-cytoplasmic male sterility, and the endosperm are major forces that strengthen the external hybridization barriers allowing, at the same time and under specific circumstances, a certain amount of gene exchange without jeopardizing the integrity of the species.     

Uses and usefulness of endosperm balance number

 The Endosperm Balance Number (EBN) hypothesis was developed in the early ’80s to explain the basis for normal seed development after intra- and inter-specific crosses, first in the potato and then in several other crop species. According to this hypothesis, each species has a genome-specific effective ploidy, the EBN, which must be in a 2 : 1 maternal to paternal ratio in the hybrid endosperm for normal development of the endosperm itself. This paper reviews how the EBN may act as a powerful isolating mechanism in sexual reproduction, maintaining the genome integrity of the species and playing an important role in the speciation of polyploids from diploids. We also provide further evidence that EBN is more important than chromosome ploidy in determining the success or failure of interspecific crosses. In fact, results from inter-ploidy and inter-EBN crosses to infuse 1EBN Solanum commersonii into 4EBN S. tuberosum demonstrated that the knowledge and manipulation of EBN is a useful tool in designing breeding schemes and in predicting the offspring ploidy and EBN. In this paper we also discuss the exceptions to the 2 : 1 EBN ratio, and report the evidence for endomitosis in the polar nuclei to explain exceptions to the EBN model in the potato. Received: 22 December 1997 / Accepted: 19 May 1998     

Pollen production and spontaneous polyploidization in diploid populations of Anthoxanthum alpinum

The most common form of polyploidization is that which occurs via the fertilization of unreduced 2 n gametes, i.e. gametes that possess the somatic chromosome number of the species. However, very few data are available concerning the frequency of spontaneous polyploidization in diploid plant populations. In this study we have quantified both the frequency of In pollen production and the frequency of polyploid seed production in diploid populations of the grass Anthoxanthum alpinum. More than 6000 seeds from four different populations collected during two years of study were screened for ploidy using a flow cytometer. In parallel, the frequency both of plants with large pollen (LP) and of LP production on these plants was quantified in two populations. No tetraploid seeds were detected, although in each population a few triploid seeds were regularly produced at a frequency of two triploid seeds per thousand. The frequency of LP producers was similar in the four populations, as was the frequency of LP production. The frequency of LP was ten times that of triploid seeds. These finding suggest that the 'triploid block' is not enough to prevent the regular occurrence of triploids in diploid populations of this species. The role played by the hypothesized 'triploid bridge' in the genesis of autotetraploids is discussed in the light of these results.     

Triploid bridge and role of parthenogenesis in the evolution of autopolyploidy

Autopolyploidization is considered to play an important role in plant evolution. In polyploidization, the polyploid evolves from the original diploid cytotype, in which the triploid state is considered to mediate the process (triploid bridge). Nevertheless, the fitness of triploid individuals seems to be too low to facilitate the polyploidization process (triploid block). The evolutionary condition of autopolyploidy was analyzed using a mathematical model focusing on the role of parthenogenesis in triploid and tetraploid individuals. In addition, offspring were assumed to arise by sexual reproduction by conjugations between haploid, diploid, and triploid gametes produced by diploid, tetraploid, and triploid individuals. According to the analysis, even if triploid block suppresses the fitness of sexually produced triploids, the polyploidization process can proceed when parthenogenesis occurs frequently. If only triploids frequently reproduce parthenogenetically, the evolutionary consequences tend to depend on the fitness of the tetraploid individuals. On the basis of a predetermined parameter set, if tetraploid fitness is relatively low, all three ploidies can coexist. Otherwise, tetraploidization occurs. In this case, triploid parthenogenesis promotes not only triploidization but also tetraploidization. However, if both triploids and tetraploids frequently reproduce parthenogenetically, the ploidy levels with the highest fitness are likely to dominate in the population through direct competition among cytotypes.     

Origins and introgression of polyploid species in Mentzelia section Trachyphytum (Loasaceae)

? Premise of the study: Polyploid speciation has been important in plant evolution. However, the conditions that favor the origination and persistence of polyploids are still not well understood. Here, we examine origins of 16 polyploid species in Mentzelia section Trachyphytum. ? Methods: We used phylogeny reconstructions based on DNA sequences from plastid regions and the nuclear gene isocitrate dehydrogenase (idh) to construct hypotheses of introgression and polyploidization. ? Key results: Molecular data suggest that homoploid hybridization has been surprisingly common in Trachyphytum. Diploid species had unequal involvement in polyploid origins, but most polyploid taxa had allopolyploid origins from extant progenitors. A few polyploids with extreme phenotypes did not appear to have extant progenitors. We infer that the progenitors of these species were derived from extinct diploid lineages or ancestral lineages of multiple extant diploids. In agreement with other recent studies, we recovered molecular evidence of multiple phylogenetically distinct origins for several polyploid taxa, including the widespread octoploid M. albicaulis. ? Conclusions: Evidence of high levels of introgression and allopolyploidy suggests that hybridization has played an important role in the evolution of Trachyphytum. Although idh sequences exhibited complicated evolution, including gene duplication, deletion, and recombination, they provided a higher percentage of informative characters for phylogeny reconstruction than the most variable plastid regions, allowing tests of hypotheses regarding polyploid origins. Given the necessity for rapidly evolving low-copy nuclear genes, researchers studying hybridization and polyploidy may increasingly turn to complex sequence data.     

Occurrence and Inheritance of 2 n Pollen in Musa

Sexual polyploidization explains the formation of most polyploidcrop species and provides means for their genetic improvement.This paper reports the occurrence and inheritance of 2n pollenin diploid and polyploidMusa . Pollen samples of male fertileaccessions available in the field gene bank of the InternationalInstitute of Tropical Agriculture were microscopically examinedthroughout a calendar year. Based on pollen diameter 2n pollenproducers were identified. The presence of 2n pollen in diploidspecies suggests that unilateral sexual polyploidization (2nxn) could have been involved in the origin of triploidMusa species.Segregation data suggests that at least one dominant gene controls2n pollen production inMusa . Further introgression of desirablealleles from diploid species to polyploids could be achievedeither through unilateral or bilateral (2nx2n ) sexual polyploidizationinMusa . Musa spp.; Musa hybrids; 2n gametes; evolution; ploidy manipulations; sexual polyploidization     

Origin of triploid Arachis pintoi (Leguminosae) by autopolyploidy evidenced by FISH and meiotic behaviour

Background and Aims

Polyploidy is a dominant feature of flowering-plant genomes, including those of many important crop species. Arachis is a largely diploid genus with just four polyploid species. Two of them are economically important: the cultivated peanut and A. glabrata, a tropical forage crop. Even though it is usually accepted that polyploids within papilionoid legumes have arisen via hybridization and further chromosome doubling, it has been recently suggested that peanut arose through bilateral sexual polyploidization. In this paper, the polyploid nature of the recent, spontaneously originated triploid cytotype of the tropical lucerne, A. pintoi, was analysed, and thereby the mechanism by which polyploids may arise in the genus.

Methods

Chromosome morphology of 2x and 3x A. pintoi was determined by the Feulgeńs technique and the rDNA sites were mapped by FISH. To investigate whether polyploidization occurred by means of unreduced gametes, a detailed analysis of the microsporogenesis and pollen grains was made.

Key Results

The 2x and 3x plants presented 9m + 1sm and a satellited chromosome type 2 in each haploid genome. Physical mapping revealed a cluster of 18S–26S rDNA, proximally located on chromosome 6, and two 5S rDNA loci on chromosomes 3 and 5. Diploid plants presented 10II in meiosis while trivalents were observed in all triploids, with a maximum of 10III by cell. Diploid A. pintoi produced normal tetrads, but also triads, dyads and monads. Two types of pollen grains were detected: (1) normal-sized with a prolate shape and (2) large ones with a tetrahedral morphology.

Conclusions

Karyotype and meiotic analysis demonstrate that the 3x clone of A. pintoi arose by autopolyploidy. The occurrence of unreduced gametes strongly supports unilateral sexual polyploidization as the most probable mechanism that could have led to the origin of the triploid cytotype. This mechanism of polyploidization would probably be one of the most important mechanisms involved in the origin of economically important species of Arachis, either by triploid bridge or bilateral sexual polyploidization.     

Trends in site-number change of rDNA loci during polyploid evolution in Sanguisorba (Rosaceae)

To elucidate the evolutionary dynamics of rDNA site number in polyploid plants, we determined 5S and 18S-5.8S-26S rDNA sites for ten species of Sanguisorba (2n=14, 28, 56) and a single species of each of three outgroup genera, Agrimonia (2n=28), Rosa (2n=14), and Rubus (2n=14) by the fluorescence in situ hybridization (FISH) method. We also estimated phylogenetic relationships among these species using matK chloroplast DNA (cpDNA) sequences, and reconstructed the evolutionary history of rDNA site number based on the maximum parsimony method. The 2n=14 and 2n=28 plants of all genera except Rosa carried two 5S rDNA sites, whereas Rosa and 2n=56 plants carried four sites. The 2n=14 plants had two 18S-5.8S-26S rDNA sites, whereas Sanguisorba annua and 2n=28 plants had four or six sites. Phylogenetic analysis showed that polyploidization from 2n=14 to 2n=28 has occurred once or three times in Sanguisorba and Agrimonia. The 5S rDNA sites duplicated during each ancestral polyploidization were evidently lost after each polyploidization. However, the duplicated 18S-5.8S-26S rDNA sites were all conserved after each polyploidization. Thus, the duplicated 5S rDNA sites tend to have been eliminated, whereas those of 18S-5.8S-26S rDNA tend to have been conserved in Sanguisorba. In the most parsimonious hypothesis, 2n=14 in S. annua is a secondary, putatively dysploid state, reduced from 2n=28.     

Glycoalkaloid aglycone accumulations associated with infection by Clavibacter michiganensis ssp. sepedonicus in potato species Solanum acaule and Solanum tuberosum and their interspecific somatic hybrids

Solanum acaule Bitt., a wild potato species, is closely related to cultivated potato (Solanum. tuberosum L.). Incorporation of desirable traits from allotetraploid [2n=4x=48, 2 endosperm balance number (EBN)] S. acaule (acl) into autotetraploid (2n=4x=48, 4EBN) S. tuberosum (tbr) is difficult due to incongruity boundaries. In this study, three hybrid combinations, each with a specific genome constitution, were produced through protoplast fusion: (1) hexaploid 2x acl (+) 4x tbr, (2) tetraploid 2x acl (+) 2x tbr, and (3) hexaploid 4x acl (+) 2x tbr hybrids. In terms of glycoalkaloid aglycones, the hybrids produced demissidine, tomatidine and solanidine, similarly to the S. acaule parental species, but S. tuberosum synthesised only solanidine. Inoculations with Clavibacter michiganensis ssp. sepedonicus (Cms), which is the causal agent of bacterial ring rot in potato, yielded significantly lower total glycoalkaloid aglycone accumulation both in S. acaule plants and in interspecific hybrids in comparison with the corresponding mock-inoculated plants. However, in S. tuberosum the aglycone levels were either higher or unchanged as a result of infection by Cms. To incorporate the desirable traits of the interspecific somatic hybrids into 4EBN S. tuberosum, sexual backcrosses were carried out. The hexaploid 4x acl (+) 2x tbr hybrids with the hypothetical 4EBN showed the greatest capacity to undergo backcrosses with S. tuberosum.     

植物多倍体研究的回顾与展望

多倍化是促进植物进化的重要力量。多倍体主要是通过未减数配子融合,体细胞染色体加倍以及多精受精三种方式起源的。其中,不减数配子是多倍体形成的主要机制。三倍体可能在四倍体的进化中起了重要作用。过去认为多倍体只能是进化的死胡同,现在发现很多多倍体类群都是多元起源的而不是单元起源的。当多倍体形成后,基因组中的重复基因大部分保持原有的功能,也有相当比例的基因发生基因沉默。多倍体通常表现出不存在于二倍体祖先的表型,并且超出了其祖先的分布范围,因为在多倍体中发生了很多基因表达的变化。主要从多倍体的起源、影响多倍体发生的因素及多倍体基因组的进化等方面回顾并展望多倍体的研究。