Capturing Heterosis in Forage Crop Cultivar Development

Abstract

Despite the widespread use of hybrid cultivars in many crop species, most forage crop cultivars are synthetic populations that do not express maximum heterosis for important agronomic traits such as yield. Several aspects of these crops, including severe inbreeding depression, perfect flowers, and polyploidy, limit the ability to produce conventional single-or doublecross hybrids, but the viable alternative of developing semihybrid populations has been relatively unexplored. Both theoretical and empirical evidence support the notion that hybrids, developed by crossing divergent populations, can outyield the better performing parental population. A method to produce semihybrids is discussed, necessitating the development and maintenance of heterotic groups within a species. Progeny formed by crossing populations are semihybrids because in a cross of two populations, 1/2 of the progeny result from interpopulation crosses and 1/2 from intrapopulation crosses. Expected genotype proportions are presented for several scenarios to show the feasibility of using semihybrids to capture partial heterotic gains. Development of pure hybrids is more problematic but could be realized by using transgenic selectable markers, such as herbicide resistances, to rogue nonhybrid plants in production fields. Molecular markers may aid in the development of the heterotic groups and in choosing parents to be crossed. Potential heterotic groups in several forage species are suggested. By requiring attention to germplasm resources, this method will also be useful for broadening and improving the genetic bases of most cultivated forage crops. "The gain in constitutional vigour, derived from an occasional cross between individuals of the same variety, but belonging to distinct families, or between distinct varieties, has not been so largely or so frequently discussed, as have the evil effects of too close interbreeding." (Darwin, 1868). F EW AGRONOMIC IMPROVEMENTS during the 20th century rival the development of hybrid maize (Zea mays L.

Assessment of the genetic variation in alfalfa genotypes using SRAP markers for breeding purposes

“…The genetic diversity level and patterns of population structure of M. sativa are required to understand the management of genetic resources in this species as well as for further progress in breeding programs. The breeding strategy is based on the partial exploitation of heterosis in alfalfa proposed for the development of semihybrids by crossing genetically divergent germplasm and identifying heterotic groups (Brummer, 1999;Milic et al, 2013;Annicchiarico et al, 2017). One potential source of a heterotic combination within an alfalfa combination would be crossing between genetically distant populations (Li and Brummer, 2012).…”

Development and Testing of Cool-Season Grass Species, Varieties and Hybrids for Biomass Feedstock Production in Western North America

“…The half-sib 4X CWR × 4X BWR hybrid populations are similar to that of semi-hybrids of grass or alfalfa [43][44][45][46] in the sense that the hybrids are comprised of genetically heterogeneous individuals, but different in that virtually all of the seeds harvested from the 4X CWR seed parent were hybrids. The semi-hybrids, in contrast, contain an equal mixture hybrid and parental populations [43][44][45][46].…”

“…The semi-hybrids, in contrast, contain an equal mixture hybrid and parental populations [43][44][45][46]. With prolific production of rhizomes and tillers, clonal production of the CWR seed parent is feasible.…”

Breeding: Incorporation of Exotic Germplasm

“…In these crops, even though substantial heterosis often occurs in the F 1 generation of exotic-byadapted crosses, pure F 1 hybrids cannot be created on a commercial scale. [8] However, production of semihybrids formed through population crosses could capitalize commercially on a large portion of the heterosis, and should present new opportunities for germplasm incorporation in open-pollinated crops. [8] …”