Orientaciones para la elaboración de un SIGC

Over recent decades, polyploidy has been considered important for crop improvement because it enhancs allele doses, allelic diver-sity, fixed heterozygosity and generates the opportunity for novel phenotypic variation that arises due to duplicated genes acquir-ing new functions (Udall and Wendel, 2006).

In this context, the following text focuses on studies conducted in the cultivated polyploid legume species.

Groundnut

Arachis hypogaea (groundnut) is a mem-ber of tribe Aeschynomeneae, subtribe Stylosanthinae, genus Arachis. Krapovickas and Gregory (1994) have described this genus as containing 69 diploid and tetra-ploid species, but recently 11 more species have been described (Valls and Simpson, 2005). The cultivated peanut, A. hypogaea is an allotetraploid (2n = 2x = 40) (Kochert et al., 1991; Halward et al., 1992; Lanham et al., 1992; Garcia et al., 1995). Arachis mon-ticola (Krapovickas and Gregory, 1994; Valls and Simpson, 2005), Arachis glabrata, Arachis pseudovillosa and Arachis nitida belonging to sections Extranervosae and Rhizomatosae are tetraploid species. It appears that there are similarities between genomes of tetraploids in sections Rhizomatosae and Erectoides and

Arachis (Stalker, 1985). Along with those spe-cies, three aneuploid species (2n = 2x = 18) (Arachis decora, Arachis palustris and Arachis praecox) are presented in this genus (Lavia, 1998). Polyploidy in these sections is believed to have occurred through independent events (Smartt and Stalker, 1982). A. hypogaea proba-bly originated from a single recent polyploidi-zation (Kochert et al., 1996; Young et al., 1996).

The allopolypoid A. hypogaea has A and B genomes, which are derived from wild spe-cies of Arachis. The diploid spespe-cies Arachis cardenasii and Arachis batizocoi are reported to have contributed the A and B genomes, respectively, in the evolution of cultivated teraploid species. However, other data (Kochert et al., 1996; Raina and Mukai, 1999) suggest that Arachis ipaensis is most likely the B genome donor to A. hypogaea (Burow et al., 2001). A genome species can be identi-fied by a cytogenetic difference on a single chromosome (Husted, 1936; Seijo et al., 2004).

However, other diploid species not having such a cytogenetic difference have been con-sidered more heterogeneous, usually being deemed to share a B genome (Moretzsohn et al., 2004). Since Arachis glandulifera does not show any homology with species hav-ing either the A or B genome, the genome of this species has been categorized into a sepa-rate class, which is known as the D genome (Stalker, 1997; Robledo and Seijo, 2008). Using RFLP (restriction fragment length polymor-phism) markers, 17 diploid species belong-ing to different sections of Arachis and three A. hypogaea accessions have been studied in order to determine the ancestral species for the A and B genomes. This suggested that Arachis duranensis and A. ipaensis contribute the A and B genome, respectively. A unique cross between these two species has resulted in a hybrid, which was followed by a rare spontaneous duplication of chromosomes for generating the cultivated allotetraploid spe-cies (Halward et al., 1991; Kochert et al., 1996;

Seijo et al., 2004, 2007). However, in contrast to this, in situ hybridization techniques used to analyse 13 A. hypogaea accessions and 15 wild species have suggested that Arachis vil-losa (A genome) and A. ipaensis (B genome) are the progenitors of A. hypogaea (Raina and Mukai, 1999; Raina et al., 2001).

Cultivated groundnut is thought to be of monophyletic origin, harbouring relatively lit-tle genetic diversity (Burow et al., 2001). Several studies show that, following duplications, cul-tivated groundnut has been isolated from its wild diploid relatives and natural introgression of alleles from wild species into cultivated spe-cies has not been demonstrated (Hopkins et al., 1999). These selective pressures have resulted in a highly conserved genome across varieties (Young et al., 1996). Molecular markers such as RAPDs, AFLPs and RFLPs showed that this iso-lation led to low nucleotide diversity in ground-nut (He and Prakash, 1997; Subramanian et al., 2000; Gimenes et al., 2002; Herselman, 2003;

Milla et al., 2005). In addition, being a natural inbreeding species, the breeding process also reduced variation (Isleib and Wynne, 1992;

Uphadhyaya et al., 2006). Therefore, develop-ment of synthetic amphidiploid in groundnut could help to broaden the genetic base, and useful genes have been introgressed from wild species to cultivated species (Burow et al., 2001). For example, synthetic amphidiploid

‘TxAG-6’ (Simpson et al., 1993) has been used in introducing root-knot nematode resistance into cultivated groundnut (Burow et al., 1996;

Simpson and Starr, 2001).

Lucerne

Medicago sativa (lucerne) is an important per-ennial food crop of the family Leguminosae, tribe Trifolieae genus Medicago. It is an out-crossing autotetraploid (Stanford, 1951), with 2n = 4x = 32 (Armstrong, 1954; Demarly, 1954), allogamous and seed-propagated (Barnes et al., 1988) and is included in the Medicago sativa complex along with diploid and tetraploid relatives. Due to the out-crossing nature of lucerne and the buffering capacity of polyploidy, it carries a high level of deleterious recessive alleles (Brouwer and Osborn, 1999). Genetic characterization of lucerne has lagged behind other major crops, due to tetrasomic inheritance and inbreed-ing depression (McCoy and Binbreed-ingham, 1988;

Mengoni et al., 2000).

Fusion between different ploidy levels of Medicago species has occurred through

asymmetric hybridization (Kuchuk et al., 1990).

Pupilli et al. (1992) reported the only symmet-ric hybrid between different levels of ploidy among Medicago species; they fused M. sativa (2n = 4x = 32) with Medicago coerulea (2n = 2x

= 16). Although these species are very similar genetically (Quiros and Bauchan, 1988), they have different ploidy levels. Therefore unre-duced gametes are necessary for sexual crosses between them (McCoy and Bingham, 1988).

Since M. coerulea and Medicago falcata belong to the ‘sativa–falcata–coerulea’ Medicago com-plex, fertilization is possible with M. sativa at the same ploidy level (Mariani and Veronesi, 1979). Most genetic maps of lucerne have been constructed in diploids because of the com-plexity of tetrasomic inheritance (Brummer et al., 1993; Echt et al., 1993; Kiss et al., 1993;

Tavoletti et al., 1996; Kalo et al., 2000). However, two genetic maps have been constructed in tetraploid populations (Brouwer and Osborn, 1999; Julier et al., 2003).

Soybean (paleopolyploid nature of the genome)

The north Asian subgenus soja has been sug-gested to be the probable wild progenitor of the cultigen Glycine max (L.) Merr. (Doyle et al., 2003). However, the soybean genome has been described as having both allo- and autopolyploid origin. An allopolyploid soy-bean genome was first hypothesized based on cytogenetic (Singh and Hymowitz, 1985) and molecular studies (Lee and Verma, 1984b;

Shoemaker et al., 1996). However, on the basis of the phylogenetic analysis of nuclear genes, its autopolyploid origin has also been hypothesized (Doyle et al., 2003; Straub et al., 2006). Although due to the absence of dip-loid progenitors or their close relatives the allopolyploid origin of soybean is not sup-ported, a novel cytogenetic approach was used to provide nearly incontrovertible evi-dence for an allopolyploid origin in soybean (Udall and Wendel, 2006). Fluorescence in situ hybridization (FISH) has also distin-guished ten chromosome pairs, suggesting that the soybean nucleus contains two dis-tinct, co-resident genomes having two types

Polyploidy 115

of centromere, presumably reflecting diver-gence in its two diploid progenitors (Udall and Wendel, 2006).

Haploid genome studies have suggested that soybean is a diploidized ancient tetra-ploid (Hadley and Hymowitz, 1973), and the high number of gene families has long sup-ported this hypothesis (Lee and Verma, 1984a;

Hightower and Meagher, 1985; Grandbastien et al., 1986; Nielsen et al., 1989; Shoemaker et al., 2002). The genetic map data of soy-bean reveal multiple nested duplications that appear to reflect an even more ancient round of polyploidy at some point in the ancestry of the genus (Shoemaker et al., 2006). It is sug-gested that the ancestral ‘diploid’ genome donors of modern ‘allopolyploid’ soybean were themselves stabilized paleopolyploids from an earlier round of genome duplication.

This nested history of cyclical or episodic polyploidy is the rule rather than the excep-tion for all plant genomes that have been investigated in detail (Udall and Wendel, 2006). Shoemaker et al. (1996) compared the relative positions of RFLP probes across nine different mapping populations of soybean and found more than 90% of the probes detected two or more hybridizing genomic fragments, and ~60% detected three or more fragments.

By comparing the markers duplicated across different linkage groups, they observed that each chromosome segment is duplicated on average 2.55 times, suggesting that one of the soybean genomes may have undergone addi-tional duplication prior to tetraploidization (Shoemaker et al., 1996; Lee et al., 1999, 2001).

A study of 256 duplicated genes identified by EST (expressed sequence tag) sequences showed that soybean has undergone at least two major rounds of duplication at approxi-mately 14.5 and 45 MYA (Blanc and Wolfe, 2004; Schlueter et al., 2004). A phylogenetic approach used by Pfeil et al. (2005) deter-mined that the ancient duplication in soybean was shared between soybean and Medicago, and probably with all of legumes approxi-mately 50 MYA.

Sequencing of BACs (bacterial artificial chromosomes) anchored by duplicated genes suggests that while the soybean genome is a diploidized paleopolyploid, an astounding amount of sequence is conserved (Schlueter

et al., 2006, 2007; Innes et al., 2008). The full genome sequence supports the numerous previous studies suggesting cyclic rounds of duplication. Schumtz et al. (2010) found that nearly 70% of the gene space still exists in multiple copy, and hypothesized the most recent duplication event to have occurred 9–13 MYA.

A number of perennial diploid rela-tives of Glycine have been found through-out Australia and Papua New Guinea, and, among these, diploid species have inter-crossed and resulted in some allopolyploid taxa (Doyle et al., 2004). Doyle et al. (2004) have defined the tomentella and tabacina com-plexes, which have been described as allo-polyploids found in the wild. These resulted from various combinations of diploid pro-genitors, which support the view that these polyploids have arisen through multiple ori-gins. Though these species are not considered food legumes, they are important indicators of the propensity for polyploidy formation in wild legumes and for generating variation for soybean improvement.

7.6 Conclusion

We must not forget that most crop legumes are actually ancient polyploids with a major duplication event shared across many genera prior to speciation approximately 54 MYA (Blanc and Wolfe, 2004; Schlueter et al., 2004;

Schumtz et al., 2010). Evidence for this dupli-cation event has been found in many legumes for which sequence resources are available.

Polyploidy across the legumes – and specifi-cally in the crop legumes – is still being inves-tigated. The Doyle Laboratory is currently working to determine ‘cryptic-polyploids’

using next-generation sequencing technolo-gies (J.J. Doyle, 2010, personal communica-tion). It is certain that the costs of sequencing will steadily continue to decrease, and that genomes of the so-called ‘orphan’ legumes will be sequenced allowing for evolutionary studies potentially to identify duplication events. What is evident is that polyploidy has played a significant role in shaping the role of many legumes as crop species.

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