Heathcliffe Riday and E. Charles Brummer

Iowa State University

xriday@iastate.edu

Abstract

In the past 25 years, alfalfa (Medicago sativa L.) yields in the United States have been stagnant using current breeding methods. Little effort has been invested into capturing heterosis in alfalfa. In this study, we report on two possible heterotic groups: M. sativa ssp. sativa and ssp. falcata. Nine elite sativa clones obtained from three commercial companies and six semi-improved or wild falcata clones were crossed in a diallel mating design. The progeny were planted in a replicated trail at two locations in Iowa in spring 1998. After two harvests in 1999, the sativa x falcata (SxF) hybrids as a group out-yielded sativa x sativa (SxS) and falcata x falcata (FxF). Even with wild falcata genotypes, SxF crosses showed high parent heterosis. The results suggest that falcata germplasm per se represents a heterotic pattern with elite sativa and, as such, should not be directly introgressed into sativa breeding programs. Rather, development of improved falcata populations would increase the frequency of desirable allelic combinations complementary to sativa, leading to high yielding hybrid alfalfa cultivars. Analysis of forage yield data showed that many SxF crosses resulted in good specific combining ability and high parent heterosis.

Introduction

Estimates of yield increases in alfalfa range from 0.15% to 0.3% per year (Hill et. al 1988, Holland and Bingham 1994). Looking at USDA data from 1919 until 1997 there has been a yield increase over that period of time. However, the increase in yield has not been strictly linear. Yields remain flat until about 1955, then increase steadily each year until the mid to late 1970’s after which yields stagnate (USDA, 1999). Wisconsin even shows a drastic decline in yields over the past 20 years (Fig. 1). Alfalfa variety tests in Ames, Iowa between 1975 and 1998 (CAIC, 1975-1998) show no increases in yield (Fig. 2); USDA data show a similar pattern for the whole state of Iowa. During the period of open pollination breeding in maize, yields were stagnant (Fig. 3). The current method of alfalfa breeding is almost exclusively recurrent phenotypic selection to choose parents which are then crossed to produce a synthetic variety (Hill 1987). Can a similar pattern be seen in alfalfa, with recurrent selection breeding doing little to improve yield, and improved cultural practices accounting for most of the yield from 1955 to about 1975?

One of the great advances of this century is the introduction of single cross hybrid in maize. After the introduction of single cross hybrids, maize yields increased by about 2% per year (Duvick, 1992) (Fig. 3). The increased yields are due to utilization of heterosis that is naturally found between particular groups in maize. Heterosis is the out-performance of progeny compared to the parents (Hallauer et al., 1988). A heterotic group is a population of genotypes that, when crossed with individuals from another heterotic group or population, consistently outperform intra-population crosses (Hallauer et al., 1988). Can similar heterotic patterns be found in alfalfa? If heterotic groups can be identified, this would suggest developing the heterotic populations separately and only combining populations when producing a semi-hybrid synthetic cultivar (Brummer, 1999).

The two main causes of heterosis are partial to complete dominance and differing allele frequencies between the two population to be crossed (Falconer and Mackay, 1996; Hallauer and Miranda, 1988; Woodfield and Bingham, 1995). One way of measuring heterosis is by separating higher order gene action from additive gene action. Combining ability analysis can partition additive gene action into general combining ability (GCA) and dominance gene action into specific combining ability (SCA) by using a diallel mating design (Falconer and Mackay, 1996). The diallel analysis is especially useful in analyzing heterosis in hybrids when parents are unavailable or unreliable for evaluation (Sprague and Tatum, 1942; Griffing, 1956). The difficulty with alfalfa is that it is a autotetraploid, so trigenic and tetragenic gene action also exist. In using a diallel analysis, with autotetraploids, GCA contains some of the dominance gene action; the majority of dominance and all of trigenic and tetragenic gene action is contained in SCA (Levings and Dudley, 1963). Studies show that trigenic and tetragenic gene action are most important in heterosis (Woodfield and Bingham, 1995; Brummer, 1999) so a diallel analysis should be sufficient to determine the magnitude of higher order gene action. If a heterotic pattern exists between two populations, on average the inter-population crosses should have a higher SCA, than intra-population crosses. The mean additive performance of the population is given by the average GCA score for the population.

Crosses between Medicago sativa subsp. sativa and subsp. falcata show heterosis (Westgate, 1910; Waldron 1920; Sriwatanapongse and Wilsie, 1968). Medicago sativa subsp. falcata is yellow flowered alfalfa that is grown in northern climates. Falcata tends to be more winterhardy then sativa and also to have more prostrate growth. Westgate (1910) examined Medicago sativa subsp. varia which is a intermediary subspecies between falcata and sativa and determined that in many cases it outperformed sativas or falcatas. Waldron (1920) reported 47.5% higher yields in sativa x falcata (SxF) crosses than crosses within the parental populations. Sriwatanapongse and Wilsie (1968) showed that sativa crosses made with ‘Kuban’ a falcata cultivar showed heterosis while the sativa x sativa (SxS) crosses showed no heterosis. In all of the above studies the authors investigated falcata as a germplasm source to be introgressed into improved sativa populations to increase yields.

Falcata is only one of many distinct alfalfa populations. Studies have shown that in some cases crosses between diverse sativa germplasm can express heterosis (Yazdi-Samadi and Stanford, 1969; Busbice and Rawlings 1974; Hill 1983). Today’s elite sativa germplasm is mostly maintained by commercial companies which have been developing distinct elite populations for the past several decades. In this study we wanted to determine if: 1) elite SxF crosses show heterosis; and 2) inter-population commercial company sat