As human societies have evolved so have the plants in the human
environment. The transition from gathering wild plants to cultivation
involved increasing interaction between humans and the plants
they used. The subsequent genetic changes in these plants resulting
in domestication of some of these cultivated species reflect
the genius of early farmers, who were the first plant breeders.
The present generation of plant breeders has tools available
that enable them to be plant engineers. In this article that
overviews the topic and the range of papers in this Special
Issue of Annals of Botany
on crop domestication, we discuss
current themes related to the genetics of crop domestication
and crop genepools that are helping to accelerate the transition
from plant breeding to plant engineering, from crop domestication
to 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 the
traits selected by early farmers that constitute the domestication
syndrome in crops, such as loss of seed shattering and increased
organ size, has been a major focus of much recent research.
Profound insights into traits associated with crop domestication
have resulted from the technologies and discoveries that make
DNA analysis and manipulation possible (for review see Phillips, 2006
These technologies are associated with PCR, development of transgenic
crops and chromosome painting, as well as DNA sequencing and
information processing. The rapid spread of communication technologies
enable new knowledge to be disseminated at a remarkable speed
worldwide and have ushered in global research initiatives related
to crop genome analysis and related research. These tools have
been used in various crops for the development of genome maps,
QTL analysis, whole-genome sequencing, fine-resolution mapping
and gene cloning. These scientific advances have also contributed
to crop improvement, with the application of, for example, marker-assisted
selection. The use of molecular techniques has provided a range
of new insights into domestication and its future course.