What evidence supports a middle-eastern origin for Brassica oleracea crops?

To date, several genetic diversity and population structure studies have been performed and published for domesticated B. oleracea, but some have been limited by low numbers of markers. More importantly, very few have included all the described B. oleracea morphotypes, and the ones that are included are often represented by a small number of accessions. What is generally lacking in these studies is data on genetic comparisons between modern hybrids and old landrace accessions.

Recently, scientists from Wageningen University analyzed the genetic diversity, genealogical relationships, and population structure among 912 accessions of B. oleracea and their wild relatives. They found that genetic diversity decreased from genebank accessions to modern hybrid accessions. Phylogenetic analysis showed evidence for two domestication lineages, the leafy head lineage (LHL) and the arrested inflorescence lineage (AIL), that arose with the onset of diversification and breeding of cabbages and cauliflowers around 400 BC in the Middle East. Different kales and wild B. oleracea were the likely progenitors of the diverse lineages (which include sprouts and kohlrabi in addition to AIL and LHL).

“Cauliflower is the least diverse morphotype and has the strongest genetic differentiation with other morphotypes, which points to a very strong genetic bottleneck. Genetic diversity reduced from landraces to modern hybrids for almost all morphotypes,” Dr. Guusje Bonnema said. This comprehensive Brassica C-group germplasm collection provides valuable genetic resources and a sound basis for B. oleracea breeding.

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Reference

Authors

Chengcheng Cai ^1,2, Johan Bucher¹, Freek T. Bakker³ and Guusje Bonnema ^1,*

Affiliations

1 Plant Breeding, Wageningen University and Research, 6708 PB, Wageningen, The Netherlands

2 Graduate School of Experimental Plant Sciences, Wageningen University and Research, 6708 PB, Wageningen, The Netherlands

3 Biosystematics Group, Wageningen University and Research, 6708 PB, Wageningen, The Netherlands

About Dr. Guusje Bonnema

Dr. Guusje Bonnema is intrigued by the enormous variation displayed by the diploid Brassica species Brassica oleracea and B. rapa. This ranges from leafy heads of cabbages, tubers of turnips and kohlrabi, curds of cauliflower and broccoli, to seedpods of oilseeds. To unravel the genetic regulation of domestication traits in diploid Brassica species, segregating populations and core collections have been developed, and genetic maps have been constructed. Recently, de novo genomes have been generated for different crop types, and a large number of B. oleracea genotypes representing different morphotypes have been resequenced. These data, combined with resequencing data from B. rapa core collections, have made it possible to detect signals of domestication for both leafy head formation and tuber formation. The results clearly demonstrate that the triplication shared by these Brassica species facilitated selection for this extreme variation, as evidence was found for subgenomic parallel selection. The focus of Dr. Bonnema’s research is the study of these domestication events and the roles played by candidate genes in generating the tubers, leafy heads, and curds of cauliflower.