Principles of Plant Genetics and Breeding

Roll call of domesticated plants
It is estimated that 230 crops have been domesticated,
belonging to 180 genera and 64 families. Some families,
such as Gramineae (Poaceae), Leguminoseae (Fabaceae),
Cruciferae, and Solanaceae, have yielded more domesticates
than others. Further, culture plays a role in the
types of crops that are domesticated. For example, the
major world tuber and root crops – Irish potato, sweet
potato, yam, cassava, and aroids – have similar cultural
uses or purposes but represent distinct taxonomic
groups. Four general periods of domestication were
proposed by N. W. Simmonds as: (i) ancient
(7000–5000 bc); (ii) early (5000–0 bc); (iii) late
(ad 0–1750); and (iv) recent (after ad 1750). Early
domesticates were made by peasant farmers who selected
and advanced desirable plants suited to their cultural
practices and food needs.
Changes accompanying domestication
Selection exerted by humans on crop plants during the
domestication process causes changes in the plants as
they transit from wild species to domesticates (Figure
2.1). The assortments of morphological and physiological
(a)
THE ART AND SCIENCE OF PLANT BREEDING 23
traits that are modified in the process and differentiate
between the two types of plants were collectively
called the domestication syndrome by J. R. Harlan.
Although the exact composition of the domestication
syndrome traits depends on the particular species, certain
basic characteristics are common (Table 2.1.). These
traits are selected at three stages in the domestication
process – seedling, reproductive, and at or after harvest.
At the seedling stage, the goal of domestication is to
get more seeds to germinate. This entails a loss of seed
dormancy as well as increased seedling vigor. At the
reproductive stage, the goal of domestication includes
a capacity for vegetative reproduction and increased
selfing rate. Plant traits modified at harvest or after the
harvest stage include elimination of seed dispersal (no
shattering), uniform seed maturity, more compact plant
architecture, and modification in photoperiod sensitivity.
Modifications targeted at the consumer include fruit
size, color, taste, and reduction in toxic substances.
The genetic control of the traits comprising the
domestication syndrome has been studied in many
crops. Generally, these traits are controlled by a few
qualitative genes or quantitative genes with major phenotypic
effects. For example, quantitative trait locus
(QTL) research has indicated that a few loci control
Figure 2.1 Tubers of domesticated tuberous species are larger and have well-defined shape, as compared to their wild
ancestors as shown in these photos of a) wild potato and b) domesticated potato. (Courtesy of Jonathan Withworth,
USDA-ARS, University of Idaho, and Peggy Bain, University of Idaho, respectively.)
(b)