Principles of Plant Genetics and Breeding

24 CHAPTER 2
Table 2.1 Characteristics of domestication syndrome traits.
General effect
Increased seedling vigor (more plants germinating)
Modified reproductive system
Increased number of seeds harvested
Increased appeal to consumers
Altered plant architecture and growth habit
traits such as flowering time, seed size, and seed dispersal
in maize, rice, and sorghum; and growth habit,
photoperiod sensitivity, and dormancy in common
bean. Furthermore, linkage blocks of adaptation traits
have been found in some species. A study by E. M. K.
Koinange and collaborators indicated that the domestication
syndrome genes in common bean were primarily
clustered in three genomic locations, one for growth
habit and flowering time, a second for seed dispersal and
dormancy, and a third for pod and seed size.
The domestication process essentially makes plants
more dependent on humans for survival. Consequently,
a difference between domesticates and their wild progenitors
is the lack of traits that ensure survival in the
wild. Such traits include dehiscence, dormancy, and
thorns. Plants that dehisce their seeds can invade new
areas for competitive advantage. However, in modern
cultivation, dehiscence or shattering is undesirable
because seeds are lost to harvesting when it occurs.
Some directions in the changes in plant domesticates
have been dictated by the preferences of consumers.
Wild tomato (Pinpenifolium) produces numerous tiny
and hard fruits that are advantageous in the wild for survival.
However, consumers prefer succulent and juicy
fruits. Consequently, domesticated tomato (whether
small or large fruited) is juicy and succulent. Thorns
protect against predators in the wild, but are a nuisance
to modern uses of plants. Hence, varieties of ornamentals
such as roses that are grown for cut flowers are
thornless.
Specific traits altered
Loss of seed or tuber dormancy
Large seeds
Increased selfing
Vegetatively reproducing plants
Altered photoperiod sensitivity
Non-shattering
Reduced number of branches (more fruits per branch)
Attractive fruit/seed colors and patterns
Enhanced flavor, texture, and taste of seeds/fruits/tubers (food parts)
Reduced toxic principles (safer food)
Larger fruits
Reduced spikiness
Compact growth habit (determinacy, reduced plant size, dwarfism)
Reduced branching
The art and science of plant breeding
The early domesticators relied solely on experience and
intuition to select and advance plants they thought had
superior qualities. As knowledge abounds and technology
advances, modern breeders are increasingly depending
on science to take the guesswork out of the selection
process, or at least to reduce it. At the minimum, a plant
breeder should have a good understanding of genetics
and the principles and concepts of plant breeding, hence
the emphasis of both disciplines in this book.
Art and the concept of the “breeder’s eye”
Plant breeding is an applied science. Just like other nonexact
science disciplines or fields, art is important to the
success achieved by a plant breeder. It was previously
stated in Chapter 1 that early plant breeders depended
primarily on intuition, skill, and judgment in their work.
These attributes are still desirable in modern day plant
breeding. This book discusses the various tools available
to plant breeders. Plant breeders may use different tools
to tackle the same problem, the results being the arbiter
of the wisdom in the choices made. In fact, it is possible
for different breeders to use the same set of tools to
address the same kind of problem with different results,
due in part to the difference in skill and experience.
As will be discussed later in the book, some breeding
methods depend on phenotypic selection. This calls
for the proper design of the field test to minimize the