1)

Almost all polyploids arise by way of unreduced gametes; other mechanisms occur, but are negligible.




2)

The most widespread and common event is (2n+n) reproduction.




3)

Triploids so produced frequently yield 4X plants on backcrossing and 6X plants on selfing.




4)

Spontaneous polyploids appear repeatedly, but their fate depends on vigor and competitive ability.




5)

The polyploids most frequently produced are probably strict autoploids, but they often lack competitive ability and may not become successfully established.




6)

Wide crosses occur in nature but not very often; if the hybrid is vigorous, it may yield a stable, fertile polyploid in the second or later generations via unreduced gametes.




7)

Wide crosses by way of (2n+n) reproduction are probably more common and successful than the above (6) pathway.




8)

The most likely pathway to a successful polyploid in nature involves neither wide crosses nor strict autoploidy but crosses between races, ecotypes and cytotypes within a biological species.




9)

The more or less autoploids so produced may generate genuine alloploids by interspecific hybridization at polyploid levels.




10)

The classic explanation of alloploidyvia “chromosome doubling” of a sterile diploid interspecific hybrid is misleading in most cases, and this route is probably negligible in the evolution of polyploid systems.

https://link.springer.com/content/pdf/10.1007/BF02860830.pdf

On Ö. Winge and a Prayer: The origins of polyploidy

Nineteen single dominant genes (R genes) for resistance to viruses, nematodes, and fungi have been positioned on the molecular map of potato using DNA markers. Fourteen of those genes are located in five "hotspots" for resistance in the potato genome. Quantitative trait loci (QTL) for resistance to late blight caused by the oomycete Phytophthora infestans, to tuber rot caused by the bacterium Erwinia carotovora ssp. atroseptica, and to root cyst nematodes have been identified on all 12 potato chromosomes. Some QTL for resistance to different pathogens are linked to each other and/or to resistance hotspots. Based on the genetic clustering with R genes, we propose that some QTL for resistance have a molecular basis similar to single R genes. Mapping potato genes with sequence similarity to cloned R genes of other plants and other defense-related genes reveals linkage between candidate genes, R genes, and resistance QTL. To explain the molecular basis of polygenic resistance in potato we propose (a) genes having structural similarity with cloned R genes and (b) genes involved in the defense response. The "candidate gene approach" enables the identification of markers highly useful for marker-assisted selection in potato breeding.

Organization of genes controlling disease resistance in the potato genome

Agriculture and fishing are the primary source of almost all our food, as well as of many other products, so remain vital activities even though their share in the economy is small and in continuous decline. Partly for this reason public policy in these areas is almost entirely integrated at European level in the Common Agricultural Policy (CAP) and Common Fisheries Policy (CFP). Forestry policies are also heavily affected by EU-level decision-making. As agriculture dominates in terms of direct contribution to GDP and numbers of people engaged in it, as well as accounting for the largest amount of public support expenditure, it is agriculture that will receive the greatest attention here.

Agriculture, Forestry and Fisheries

Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating disease for potato cultivation. Here, we describe the positional cloning of the Rpi-blb1 gene from the wild potato species Solanum bulbocastanum known for its high levels of resistance to late blight. The Rpi-blb1 locus, which confers full resistance to complex isolates of P. infestans and for which race specificity has not yet been demonstrated, was mapped in an intraspecific S. bulbocastanum population on chromosome 8, 0.3 cM from marker CT88. Molecular analysis of a bacterial artificial chromosome (BAC) clone spanning the Rpi-blb1 locus identified a cluster of four candidate resistance gene analogues of the coiled coil, nucleotide-binding site, leucine-rich repeat (CC-NBS-LRR) class of plant resistance (R) genes. One of these candidate genes, designated the Rpi-blb1 gene, was able to complement the susceptible phenotype in a S. tuberosum and tomato background, demonstrating the potential of interspecific transfer of broad-spectrum late blight resistance to cultivated Solanaceae from sexually incompatible host species. Paired comparisons of synonymous and non-synonymous nucleotide substitutions between different regions of Rpi-blb1 paralogues revealed high levels of synonymous divergence, also in the LRR region. Although amino acid diversity between Rpi-blb1 homologues is centred on the putative solvent exposed residues of the LRRs, the majority of nucleotide differences in this region have not resulted in an amino acid change, suggesting conservation of function. These data suggest that Rpi-blb1 is relatively old and may be subject to balancing selection.

An ancient R gene from the wild potato species Solanum bulbocastanum confers broad-spectrum resistance to Phytophthora infestans in cultivated potato and tomato.

Ecological factors, hybrid sterility and differences in ploidy levels are well known for contributing to gene-flow barriers in plants. Another common postzygotic incompatibility, hybrid necrosis, has received comparatively little attention in the evolutionary genetics literature. Hybrid necrosis is associated with a suite of phenotypic characteristics that are similar to those elicited in response to various environmental stresses, including pathogen attack. The genetic architecture is generally simple, and complies with the Bateson-Dobzhansky-Muller model for hybrid incompatibility between species. We survey the extensive literature on this topic and present the hypothesis that hybrid necrosis can result from autoimmunity, perhaps as a pleiotropic effect of evolution of genes that are involved in pathogen response.

Hybrid necrosis: autoimmunity as a potential gene-flow barrier in plant species

In this article the term ‘pollinators’ refers to diploid genotypes of Solanum tuberosum Group Phureja which are used to pollinate autotetraploid potatoes for the induction of autonomous growth of unfertilized egg cells. Plants obtained from such egg cells contain chromosomes of the female only and are called dihaploids. The successful selection of ideal pollinators from Group Phureja is described. A pollinator is considered to be ideal, if it has good male fertility, homozygosity for the dominant seed marker embryo-spot and the ability to induce in autotetraploid cultivars a high number of dihaploids both per berry and per 100 seeds. Embryo-spot is a deep purple or reddish coloration at the base of the cotyledons of the embryo, visible on both sides of the flat seeds. It is one manifestation of a set of complementary genes with pleiotropic action, causing a concentration of anthocyanins at the base of all plant organs that are homologous to leaves. The complementary genes are: a basic gene P for anthocyanin production (this is the seedling marker ‘coloured hypocotyl’) and allele B
d of locus B. Thirty-three homozygous B
d
B
d
PP plants and 41 B
d
bPP plants were identified and a number from both classes tested as pollinator onto varieties. Among the homozygous group a number of ‘ideal pollinators’ could be detected. Unexpectedly a few polyploids grew from spotless seeds when using homozygous pollinators on varieties; 1.4% showed purple hypocotyl and 2.6% were matroclinous. The latter group of polyploids probably may have arisen from ‘unreduced’ egg cells of the cultivars used. In the progenies from crosses between recessive cultivars (bbbbpppp) and heterozygous pollinators (B
d
bPP) the ratio of spotted: spotless hybrid seeds appeared to deviate greatly from 1:1. This is ascribed to ‘unreduced’ gametes, which originate during meiosis, presumably through lack of formation of the ‘reductional’ (= first division) cell wall. On this assumption a map distance of 30 and 24 cross-over units is estimated for locus B on the basis of the proportion of nulliplex tetraploid hybrids in two different crosses. High- and low-seed-set pollinators were found at a ratio of 8:21 suggesting monogenic dominance of low seed-set. Seeds per berry and haploids per berry are significantly correlated using low-seed-set pollinators on cultivars, but they are not when pollinators are used that produce high numbers of seeds per berry. A hypothesis is put forward to explain this phenomenon.

Selection from Solanum tuberosum group phureja of genotypes combining high-frequency haploid induction with homozygosity for embryo-spot

Solanum bulbocastanum (2n=2x=24) has valuable characters for potato breeding, but cannot be hybridized directly with S. tuberosum cultivars. Both S. acaule (2n=4x) and S. phureja (2n=2x) were used as bridging species. Triploid S. acaule × S. bulbocastanum were doubled with colchicine and the resulting fertile hexaploid F1's crossed with S. phureja. The triple hybrids obtained were tetraploid or nearly so. The two genomes of S. acaule in these triple hybrids probably pair preferentially, which may provoke pairing and possibly crossing over between the chromosomes of S. bulbocastanum and S. phureja. More than 20000 pollinations of the triple hybrids with four potato cultivars had to be made to produce 40 quadruple hybrids. These highly vigorous hybrids varied greatly in many morphological characters, resistance to Phytophthora infestans, fertility and crossability. The chromosome numbers are 48 (24 hybrids), 49 and 46, but some higher ploidy levels (65, 66, 72 chromosomes) were found as well. Their origin is to be sought in the fusion of an unreduced egg cell from triple hybrids (either euploid or hypoploid) and a reduced male gamete from the cultivars. This view is corroborated by their extreme resistance to Phytophthora. Also some 48-chromosome hybrids are highly resistant, which may indicate introgression from S. bulbocastanum. Most quadruple hybrids are readily inter-crossable and crossable as females with cultivars; several also as males. Two could be hybridized with S. bulbocastanum, but the few seeds dit not germinate. Studies of pachytene stage of meiosis revealed the presence of a S. bulbocastanum chromosome in at least one tetraploid hybrid, which is highly resistant to Phytophthora. At metaphase I of meiosis chromosome associations higher than quadrivalents were not found. Except in one hybrid, the frequency of quadrivalents did not exceed one per cell and the average proportion of chromosomes associated as bivalents amounted to 90%. The quadruple hybrids (‘double-bridge’ hybrids) appear good starting material for breeding programmes aimed at introducing genes from S. bulbocastanum into S. tuberosum cultivars.

Double-bridge hybrids of Solanum bulbocastanum and cultivars of Solanum tuberosum

Hybrid necrosis is the premature gradual death of leaves and leaf sheaths in certain wheat hybrids It is based on two complementary genes Ne
1 and Ne
2 Hitherto 89 Ne
1-carriers and 207 Ne
2-carriers from many countries are known (table 1 and 2) This corresponds with more than 18,000 different necrotic cross combinations The degree of necrosis in the F1's varies greatly, due to multiple allelism of Ne
1 and Ne
2 Nine necrosis grades are described in table 3 Grades 6–8 are called severe necrosis (no seed is produced), 3–6 moderate necrosis (premature seed), 0–3 weak necrosis (normal seed) Most severely necrotic F1's could be brought to seed production by raising them under extra favourable conditions in a small growth-chamber Growing moderately necrotic F1's in pots outside, at a high fertility level of the soil, resulted in a striking increase of the seed production On the basis of the above mentioned data and results some practical advice is given to the breeders about the handling of necrotic crosses In addition some ways to preclude hybrid necrosis are indicated, though the wheat breeder is advised against a systematic avoiding of necrosis because many combinations of excellent varieties then remain untried

Hybrid necrosis as a problem for the wheat breeder

Hybrid necrosis in wheat is based on 2 complementary genes Ne
1 and Ne
2. Unitl now among the 509 varieties and selections tested 89 Ne
1-and 207 Ne
2-carriers were found. By tracing the descendence of these carriers several sources of the necrosis genes (mostly land varieties) could be detected. At the same time it was demonstrated that notably the frequent use of certain carrier-varieties as parents in crosses has promoted the distribution of the necrosis genes.

Sources and distribution of the complementary genes for hybrid necrosis in wheat

The results of extensive crosses between the non-tuberous species Solanum brevidens and S. etuberosum on the one hand and ten tuber-bearing Solanum species on the other are presented. Three crosses gave rise to viable progeny. Two progenies consisted of diploid plants only of the strictly self-incompatible species of the mother parent. One cross, viz. S. etuberosum × S. pinnatisectum, produced highly vigorous but fully male sterile F1 hybrids. It is suggested that this hybrid together with those between the tomato, Lycopersicon esculentum, and S, pennellii and S. lycopersicoides constitute piers of a bridge between tomato and potato species which in the future might enable gene transfer between these two crops via their wild relatives. However, such idea has to be treated with all proper reserve. The production of this new hybrid is the first step in making accessible to potato breeding the valuable genes which have been detected in S. brevidens and S. etuberosum, viz. the genes for high resistance to frost, leafroll and Y-virus.

J. G. Th. Hermsen

Papers

Selection from Solanum tuberosum group phureja of genotypes combining high-frequency haploid induction with homozygosity for embryo-spot

Double-bridge hybrids of Solanum bulbocastanum and cultivars of Solanum tuberosum

Hybrid necrosis as a problem for the wheat breeder

Sources and distribution of the complementary genes for hybrid necrosis in wheat

Successful hybridization of non-tuberous Solanum etuberosum Lind. and tuberbearing S. pinnatisectum Dun.