REPRODUCTIVE-BIOLOGY IN SUBFAM MALOIDEAE (ROSACEAE)

Maloids are versatile in reproductive mode-vegetative spreading, self-incompatibility, self-compatibility, and apomixis all occur-and they engage in extensive hybridization and polyploidy. Naturally occurring gametophytic apomixis has been reported from almost 100 taxa in six genera-Aronia, Amelanchier, Cotoneaster, Crataegus, Malus, and Sorbus s.l. Generally in maloids apomixis is facultative, aposporous, pseudogamous, and characterized by a distinctive multiplicity of megagametophytes per ovule. Unlike their sexual relatives apomicts are almost always triploid or tetraploid and self-compatible. Maloid polyploids that have been studied express some apomixis. Reported evidence for apomixis includes ploidy level and mode of development of megagametophytes, presence of multiple megagametophytes in one ovule, polyembryony, progeny of maternal phenotype, complete pollen sterility, and seed set in the absence of pollination. Crosses between apomictic and sexual taxa yield apomictic offspring in most cases, indicating, along with other studies, that apomixis is genetically dominant over sexuality in the Maloideae. Hybridization at generic and specific levels is remarkably frequent. Smaller genera tend to be diploid; larger genera also contain triploids and tetraploids; and higher ploidy levels and aneuploidy are rare. Hybridization, often coupled with polyploidy and uniparental reproduction, leads to new, more or less reproductively isolated forms in the Maloideae.