8. Thesis structure
1.16 Gene action and inheritance of bruchid resistance
Genes, comprised of DNA (deoxyribonucleic acid), are the basic units of inheritance.
Gene action is the functioning of a gene in determining the phenotype of an individual.
Gene action can be grouped into two categories, additive and non-additive. The non- additive gene expression may exhibit dominance, recessivity, no dominance, over- dominance and epistasis (Acquaah, 2007).
The effect of gene expression on phenotype is generally thought of as being either additive or non-additive (Falconer, 1981). Additive gene action occurs when the phenotypic effect of one gene adds to the phenotypic effect of another gene i.e., each of the two alleles contributes equally to the production of qualitative phenotypes; neither allele is dominant. The heterozygous genotype produces a phenotype that is intermediate between those produced by the homozygous genotypes. Traits affected by additive gene action are moderately to highly heritable and will be affected very little by outcrossing and inbreeding (Kearsey and Pooni, 1996). This type of gene action influences many of the traits a breeder is interested to select for in a breeding programme. Improvement through selection can be made effective when additive gene effects, which are fixable, are involved in conferring bean resistance to bruchids. When the gene expression is non-additive, the phenotypic expression of one gene does not necessarily add to the phenotypic expression of another gene. Non-additive gene action is observed when the additive model inadequately explains the variance (Falconer, 1981).
Epistasis is a form of non-additive gene action. It is the interaction between the genes at two or more loci, so that the phenotypic expression is masked. In epistatic interaction one gene may control the degree to which another gene is expressed. In quantitative traits, epistasis is described as non-allelic gene interaction. Epistasis is important in population genetics when the fitness effects of a genotype depend on what genotype it is associated with at the other locus. It is in this situation that natural selection can maintain linkage disequilibrium in a population (Hill et al., 1998; Acquaah, 2007).
In plant breeding, studies of hybrid populations sometimes report the presence of traits or phenotypes that are extreme, relative to either of the parental lines (Rieseberg et al.,1999). The generation of these extreme phenotypes in hybrids (i.e. phenotypes that exceed those of either parental line) is referred to as transgressive segregation (Grant, 1975; De Vicente & Tanksley, 1993). Transgressive segregation has been hypothesized as an important mechanism by which novel adaptations can arise in hybrids.
There are many mechanisms that could be responsible for transgressive segregation in hybrids such as: an elevated mutation rate reduced developmental stability, epistatic effects between alleles, overdominance caused by heterozygosity at specific loci or chromosome number variation (Xu et al., 1998).
A recent review of phenotypic variation in hybrids indicates that transgressive segregation occurs frequently in segregating plants (Rieseberg et al., 1999). Out of 171 studies reviewed, 155 (91%) reported at least one transgressive trait and 44% out of 1229 traits examined were transgressive. Transgressive segregation was found to be more frequent in plants than in animals, in intraspecific than in interspecific crosses, in inbred than in outbred populations and in domesticated than in wild populations (Darlington and Mather, 1949; Vega and Frey, 1980; Rieseberg et al.,1999). Rick and Smith (1953) proposed three potential explanations for the occurrence of interspecific transgression including de novo mutation induced by hybridity, complementary action of genes from the two parental species and unmasking of recessive genes normally held heterozygous. However, genetic studies indicate that transgressive segregation mostly results from the appearance, in individual genotypes, of combinations of alleles from both parents that have effects in the same direction: complementary gene action (De Vicente & Tanksley, 1993; Rieseberg et al., 1999).
There are many genes in plants without known effects besides the fact that they modify the expression of a major gene by either enhancing or diminishing it. These are known as modifying genes (Bhatnagar et al., 2004). The effect of modifier genes may be understated, but they are very important because they influence phenotypic expression.
These trait modifications are of concern to a plant breeder as they conduct breeding programmes to improve traits that involve many major traits of interest. To develop an efficient and successful resistance breeding programme, understanding the genes controlling resistance is fundamental. Information from the literature on bruchid
resistance inheritance studies is scanty. Arcelin was shown to be inherited as a single dominant gene and resistance to Z. subfasciatus was easily transferred to commercial bean types (Cardona, et al., 1990). However, the monogenic inheritance of arcelin leaves open the possibility that biotypes of Z. subfasciatus may exist or evolve, which are able to overcome this resistance. In a study to understand the inheritance of resistance to A. obtectus, Kornegay and Cardona (1991) found that resistance was inherited in a recessive manner when crossed with two commercial cultivars.
In a study where 10 inbred lines of maize were evaluated to determine combining ability for weevil resistance, Kang et al. (1995) found that additive gene effects were more important than non-additive gene effects. In another study to investigate inheritance of resistance to oviposition by maize weevil, Tipping et al. (1989) reported that additive gene action was important. Derera et al. (2001a, b) investigated gene action for weevil resistance in both free-choice and no-choice tests and found significant additive, non- additive and maternal effects. Dhliwayo et al. (2005) reported that both general combining ability (GCA), which is defined as average performance of individual lines in crosses and specific combining ability (SCA), which is the deviation of some crosses from the expected value (sum of the GCA values of the two parents involved)1, were also found to be important for resistance to maize weevil. Dhliwayo and Pixley (2003) reported the importance of reciprocal effects on maize weevil resistance. Cockerham (1963) suggested partitioning of reciprocal effects into maternal and non-maternal effects. This is useful in determing whether maternal or extranuclear factors are involved in the expression of a trait.
Variation in an individual’s phenotype may be determined, not only by the genotype and environment of that individual, but also by maternal effects i.e., the contribution of the maternal parent to the phenotype of its offspring beyond the equal chromosomal contribution expected from each parent (Roach and Wulff, 1987). During angiosperm development, multiple fertilization occurs where one sperm nucleus fuses with the egg nucleus to form a zygote. The other sperm nucleus fuses with the two polar nuclei to from triploid (3n) endosperm nucleus. Although the endosperm is not always triploid, it always contains more doses of maternal than paternal genes (Huidong, 1987; Bogyo et al., 1988). As a consequence of the differential dosage of male and female genes, the
1Kearsey and Pooni (1996)
female parent may have a more important role in determining the characteristics of the trait under study.
Genetic analysis of grain resistance to weevils is reportedly complicated (Widstrom, 1989; Serratos et al., 1997) because the weevil feeds on diploid and triploid tissues, which are both maternal and biparental in origin due to the fact that grain tissues belong to two different generations and have different gene doses from the parents (Serratos et al., 1997). While the endosperm persists in the mature seeds of maize grain, in dry beans the 3n endosperm disappears (absorbed by the embryo) during seed development and the cotyledon, and not endosperm, becomes food storage.
Maternal effects are considered sources of error because generally they are non- Mendelian and reduce the precision of genetic studies (Roach and Wulff, 1987). The consequences of maternal effects for the response to selection may be further complicated by the correlation between maternal and offspring environments (Singh and Murty, 1980; Rossiter, 1996). Actual influence of maternal effects on response to selection will depend on the type of maternal effect involved. Environmental maternal effects will increase the amount of environmental “noise” and thus slow the response to selection (Alexander and Wulff, 1985). On the other hand, cytoplasmic or nuclear genetic maternal effects inflate the response to selection if maternal effects are dominant (Naylor, 1964). Although Cockerham (1963) indicated that maternal effects in plants were minimal and did not generally require consideration, Roach and Wulff (1987) provided substantial evidence that maternal effects have significant effects on the phenotype of an individual.