USING MOLECULAR ALLELIC VARIATION TO UNDERSTAND DOMESTICATION PROCESSES AND CONSERVE DIVERSITY IN BRASSICA CROPS
2003; International Society for Horticultural Science; Issue: 598 Linguagem: Inglês
10.17660/actahortic.2003.598.26
ISSN2406-6168
Autores Tópico(s)Plant Molecular Biology Research
ResumoThe harvested curd of cauliflower (Brassica oleracea var. botrytis L.) consists of proliferating, arrested inflorescence and floral meristems. The origins and events leading to the domestication of this important crop trait remain unclear. A similar phenotype observed in the ap1-1/cal-1 mutant of Arabidopsis thaliana resulted in speculation that the orthologous genes from B. oleracea may be responsible for this characteristic trait. A recent molecular and genetic study has led to development of a genetic model based on segregation of recessive alleles at specific, mapped loci of the candidate genes BoCAL and BoAP1. This model, although incomplete, accounts for differences in stage of arrest between cauliflower and Calabrese broccoli (B. oleracea var. italica Plenck), and predicts the intermediate stages of arrest similar to those observed in Sicilian Purple types. There appears to be a reduction in allele diversity at two key loci associated with this crop type. Surveys of ex situ genetic resource collections have demonstrated the association of alleles of BoCAL-a with curding phenotypes of B. oleracea. Strong correlations exist between specific alleles of BoCAL-a and discrete inflorescence morphologies. These complementary lines of evidence suggest that the cauliflower curd arose in southern Italy from a heading Calabrese broccoli via an intermediate Sicilian crop type. The close association of BoAP1-a and BoAP1-c with the self-incompatibility locus S may have contributed to the observed reduced number of S-alleles within the modern cauliflower gene-pool. The results indicate that it is important to consider the effect of strong selection for crop type on genetic erosion of loci linked in coupling to genes underlying the crop phenotype. INTRODUCTION The assessment and conservation of biodiversity in the context of crop species is currently loosely defined in terms of both genetic resources and the underlying genetic diversity. For conservation and utilisation of genetic diversity it is useful to be able to distinguish between a) allelic variation at one or more loci and b) unique combinations of alleles which result in unique or distinct phenotypes, or c) the ability of populations to undergo rapid adaptation. The clear definition of these different aspects of genetic diversity is particularly important when considering strategies and techniques for collection, evaluation, and conservation. This paper will outline an example of how variation at a small number of loci requires careful interpretation in the context of crop domestication. The characteristic curd phenotype of cauliflower consists of proliferating, arrested, inflorescence meristems. This phenotype is seen in a wide range of forms across the crop types defined as Brassica oleracea L. vars. botrytis and italica, including the convars Romanesco, DiJesi, Macerata, and the classic white cauliflower. Each of these types is closely associated with eco-geographic distribution in Italy, which is the centre of diversity for the crop (Massie et al., 1996). Curd appearance differs significantly across the range of morphotypes (Watts, 1965), dependant to a large extent on differing iteration intervals during curd development (Kieffer et al., 1998). Most curd forms are heading, with the notable exception of white sprouting broccoli (Gray, 1982). Colour is the second most distinguishing factor between curds of different convars. Thus, in addition to the Proc. IS on Sust. Use Of Plant Biodiv. Eds. E. Duzyaman Y Crisp and Tapsell, 1993; Massie, 1998). As with all historical sources however, meaning can be lost during translation and interpretation. Even picture sources are scarce and little of what could be positively identified as a cauliflower exists before the sixteenth century (Zeven, 1996). There are conflicting theories of cauliflower phylogeny, although there is widespread support for the likelihood of the classic curd phenotype of cauliflower originating within the last two thousand years (Crisp, 1982; Helm, 1963; Thompson, 1976). The most prevalent domestication theory appears to be of an Eastern European origin where both B. cretica (Gates, 1953) and B. cypria (Giles, 1941) have been proposed as the precursor of the cauliflowers. Following introduction into Italy, possibly around 1490 (Thompson, 1976), a secondary centre of diversity arose with development of distinct local forms isolated by geography and strong trade barriers between the many feudal states (Massie, 1998). Classical breeding investigations have attempted to characterise the genetics underlying the curding phenotype of cauliflowers, with limited success (Crisp, 1982; Pease, 1926). Crosses between cauliflower and Calabrese broccoli often produce intermediate phenotypes, suggesting curding is a multifactorial trait. However, segregation studies suggest that, although polygenic, curding is more likely to be controlled by a few key loci rather than through an accumulation of many ‘less significant’ mutations (Crisp, 1982). Until recently a simple genetic model accounting for the inflorescence phenotype of the Brassica cauliflower had not been proposed. CANDIDATE GENES FROM ARABIDOPSIS HAVE LED TO A GENETIC MODEL FOR CURD FORMATION An Arabidopsis mutant was identified whose inflorescence structure was enhanced from an apetala1-1 phenotype (Irish and Sussex, 1990) to a form strongly resembling the curd phenotype of the Brassica cauliflower (Bowman et al., 1993). The mutant gene, which in conjunction with APETALA1 (AP1), is responsible for this phenotype was named CAULIFLOWER (CAL). These two genes are closely related members of the MADS-box gene family of putative transcription factors, and are associated with both floral organ identity and the switch from inflorescence to floral meristem fate. A B. oleracea orthologue of the Arabidopsis CAL gene, BoCAL (syn. BoCAL-a), was cloned Kempin et al. (1995), and an allele of this gene from the Brassica cauliflower, termed BobCAL (syn. Bocal-a), was found to contain a premature stop codon. The similarity between the ap1-1/cal-1 double mutant Arabidopsis and the Brassica cauliflower curd, along with the discovery of the ‘mutated’ copy of BoCAL in B. oleracea var. botrytis, suggested that the curding phenotype of cauliflower may be at least in part due to a mutation at a locus containing BoCAL. A simple genetic model for domestication of cauliflower curds has subsequently been proposed (Smith and King, 2000). The model is based on segregation of recessive alleles at specific, mapped loci of the candidate genes BoCAL and BoAP1. This accounts for differences in stage of arrest between cauliflower and Calabrese broccoli (B. oleracea var. italica Plenck), and predicts the intermediate stages of arrest similar to those observed in Sicilian Purple types. Although the model is incomplete in accounting for all stages of developmental arrest subject to environmental variation, it does provides a useful basis in
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