Quantitative traits show continuous phenotypic variation in a population due to combined effects of genes and environments. Examples of quantitative traits in tall fescue include digestibility, forage yield, and drought stress tolerance. Improvement of quantitative traits is an important goal of many breeding programs. Analysis of QTL has been a major area of genetic study for many decades. Traditionally, study of quantitative traits largely depended on statistical approaches based on means, variances, and covariances of relatives. Thus, the biological nature of the quantitative traits in terms of number of genes involved and their chromosomal location was largely unknown. With the advent of molecular markers and specific statistical methods, it became possible to follow the segregation of quantitative traits via linked markers (Tanksley, 1993). Mapping markers linked to QTLs identifies regions of the genome that contain gene(s) involved in the expression of quantitative traits.
Efforts are underway to apply molecular markers for the genetic improvement of quantitative traits of tall fescue. Two examples include increased digestibility and improved drought stress tolerance. Phenotypic data, including in vitro dry matter digestibility (IVDMD), have been collected from the mapping population described by Saha et al. (2005) for three consecutive years. These data are being evaluated in conjunction with SSR marker data to identify possible QTLs associated with IVDMD. These QTL will be evaluated subsequently for their effectiveness in a selection scheme aimed at improving IVDMD of tall fescue. Database and literature searches have identified a number of genes that appear to be important for osmotic stress (drought, cold, salt, abscisic acid, and oxidative stress). The majority of these genes originate from the model plant Arabidopsis thaliana. Sequences from these genes are being screened to identify corresponding molecular markers in tall fescue. These markers then will be evaluated for their association with drought stress tolerance in tall fescue. Those markers that are strongly correlated with stress tolerance can be used subsequently in breeding improved tall fescue cultivars.
A prerequisite for marker-assisted breeding is a robust set of informative markers for the species of interest. Location of QTLs can be identified on genetic linkage maps and associated markers can be used in marker-assisted selection to improve economically important traits of tall fescue (Xu et al., 1995). An efficient way to accelerate development of disease resistant cultivars is marker-assisted selection (Frisch et al., 1999). Identification of markers that are linked to the genes of interest is very important. Marker-assisted selection was used to pyramid disease resistance genes in rice (Huang et al., 1997). With appropriate markers, a large number of plants can be screened quickly at an early stage, thereby reducing experimental size, labor requirements, and time.
Microsatellite markers associated with forage digestibility in tall fescue were identified. Seven markers associated with high digestibility and six others linked to low digestibility were used to initiate marker-assisted breeding. These markers contributed 2.3 to 10.1% effect on forage digestibility as estimated using IVDMD. It would be useful to select for alleles leading to increased digestibility while at the same time eliminating alleles contributing to decreased digestibility. It is estimated that a 1% increase in IVDMD can lead up to a 3.2% increase in daily animal live-weight gains (Casler and Vogel, 1999). Thus, use of these markers might increase the forage digestibility in resulting populations. The population developed will be evaluated with populations based on phenotypic selection (IVDMD analysis) and combination of marker and phenotypic selection to determine the effectiveness of marker-assisted selection.