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- From Crop Domestication to Super-domestication

As human societies have evolved so have the plants in the humanenvironment. The transition from gathering wild plants to cultivationinvolved increasing interaction between humans and the plantsthey used. The subsequent genetic changes in these plants resultingin domestication of some of these cultivated species reflectthe genius of early farmers, who were the first plant breeders.The present generation of plant breeders has tools availablethat enable them to be plant engineers. In this article thatoverviews the topic and the range of papers in this SpecialIssue of Annals of Botany on crop domestication, we discusscurrent themes related to the genetics of crop domesticationand crop genepools that are helping to accelerate the transitionfrom plant breeding to plant engineering, from crop domesticationto crop super-domestication.

Cultivated or domesticated plants, when mankind propagates,plants and harvests them, have played significant roles in manyof the advances that pure and applied botanical sciences havemade in the last few centuries. The earliest farmers recognizeduseful genetic variation that could be chosen from the wild,planted, harvested and reselected in order to gradually developimproved populations with a range of desirable traits. The domesticatedforms bore only limited resemblance to their wild ancestorsdue to the selection of domestication genes. Early in the 18thcentury, the first conscious hybridization of plants occurredusing the cultivated ornamental species Dianthus (Phillips, 2006).Knowledge of plant hybridization paved the way for the use byplant breeders of intra- and interspecific hybridization incrop improvement. Mendel's discovery of the laws of genetics,based most famously on his experiments with the domesticatedgarden pea, led to improved understanding of the variation indomesticated and wild species that so impressed Darwin (1859).Subsequently, sophisticated crop breeding programmes were developedthat enabled more efficient introduction of desirable traitsfrom one cultivar to another. Knowledge of the evolutionaryrelationships between crops and their wild progenitors has facilitatedmore efficient exploitation of the genetic resources representedby the wild relatives of domesticated species (for a review,see Hajjar and Hodgkin, 2007). Currently, domesticated cropssuch as rice, maize and tomato are major targets of studiesin molecular genetics. Research on domestication-related traitsis leading to a better understanding of how genetic controlof phenotypic differences is effected. For example, in determiningthe extent to which similar (orthologous) genes are involvedin producing similar phenotypes in distantly related speciesand working out how to transfer desirable genes between speciesthat cannot be hybridized sexually, and then how to controlthe expression of the genes once they are transferred.

Hybridization and selection have both been involved in the originof crops and the process of domestication since early times.Bread wheat, which is a hexaploid (2n = 6x = 42 chromosomes),arose through fortuitous hybridization between tetraploid wheat(Triticum turgidum ssp. dicoccum, 2n = 4x = 28) and diploidgoatgrass (Aegilops tauschii, 2n = 2x = 14), a weed of earlywheat fields. New groups of bananas and plantains developedwhen diploid domesticated bananas (genome AA) spread into therange of wild Musa balbisiana (genome BB), producing the AABand ABB triploids (see Heslop-Harrison and Schwarzacher, 2007).Modern strawberries (Fragaria x ananassa) are a consequenceof hybridization between North American F. virginiana and SouthAmerican F. chiloensis when these hitherto geographically isolatedspecies were cultivated in close proximity in European gardens(Bringhurst and Voth, 1984). Selection, both intentionally byhumans (conscious selection) and as a result of environmentalfactors (natural or automatic selection) has, in most crops,established the traits associated with the domestication syndrome(Hammer, 1984). Weed species have co-evolved with crops andhave been under similarly intense selection pressures. Weedcontrol through agronomy and the better competitive abilityof crops has been continuous over the 10 000 years of agriculture,but weeds still reduce yields and contaminate crops. In rice,changing agricultural practices is leading to the emergenceof new weedy rice forms in the last decade (Cao et al., 2006).Thus, as with the crops, weeds are also evolving rapidly underselection.

Analysis of the genetic, genomic and molecular basis of thetraits selected by early farmers that constitute the domesticationsyndrome in crops, such as loss of seed shattering and increasedorgan size, has been a major focus of much recent research.Profound insights into traits associated with crop domesticationhave resulted from the technologies and discoveries that makeDNA analysis and manipulation possible (for review see Phillips, 2006).These technologies are associated with PCR, development of transgeniccrops and chromosome painting, as well as DNA sequencing andinformation processing. The rapid spread of communication technologiesenable new knowledge to be disseminated at a remarkable speedworldwide and have ushered in global research initiatives relatedto crop genome analysis and related research. These tools havebeen used in various crops for the development of genome maps,QTL analysis, whole-genome sequencing, fine-resolution mappingand gene cloning. These scientific advances have also contributedto crop improvement, with the application of, for example, marker-assistedselection. The use of molecular techniques has provided a rangeof new insights into domestication and its future course.

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