Breeding Self-Fertilizing Plants: From Inbred to Hybrid Cultivars

Abstract

Pollen falls on to the stigma of the same flower in self-fertilizing (selfing) species, which are often annuals with little plant height, small flowers, short bud development time and flower longevity, and small seed sizes. Such associations evolved as a result of a strong r selection that maximizes population growth rate (Snell and Aarssen 2005) by producing many offspring with low surviving probability to adulthood. Most selfing species show hermaphrodite flowers and in some outcrossing occurs up to 5 %, which may be affected by humidity, temperature, and location. There are various mechanisms promoting selfing, for example, cleistogamy ensures complete selfing because foreign pollen cannot reach the stigma of a closed flower that does not open at all. Flowers open but after pollination in some small grain cereals such as rice or wheat, or the anthers surround closely the stigma in some crops of the nightshade family such as tomato. Selfing leads to homozygosity, but selfing species do not show inbreeding depression but may exhibit heterosis. Hence, selfing species cultivars may be inbred lines or hybrids. Composites and multilines are two other cultivar types. The former includes closely related lines such as isogenic (or isolines), while the latter may comprise inbred lines, hybrids, and populations sharing common traits. The methods for crossbreeding selfing species are mass selection, pedigree, bulk, single seed-descent, backcrossing, hybridization, and population improvement through recurrent selection. Plant genetic engineering, DNA marker-assisted breeding, and genomic selection may also be used for breeding selfing species.