Apomixis, Patterns of Morphological Variation, and Species Concepts in subfam. Maloideae (Rosaceae)

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Abstract
Asexuality and self-fertilization weaken the correlation between morphological, reproductive, and genetic units. The pattern and tempo of evolution may thus differ fundamentally between uniparentally reproducing groups and outbreeders. Apomixis, which is widespread in angiosperms, is reported from about 90 species from five of the largest genera of the Maloideae. A range of breeding systems occurs in these genera including varying degrees of sexuality, self-incompatibility and self-compatibility, and pseudogamy as well as different ploidy levels and interspecific hybridization. Some apomictic members of the subfamily display greater morphological variation between populations than they do between individuals within a population, a pattern often attributed to uniparental reproduction. Because maloid apomicts are generally self-compatible, self-fertilization may also contribute to low intrapopulation morphological variability, although selfing appears to be less important than apomixis. In the two genera for which there are the most data concerning apomixis and patterns of morphological variation, Amelanchier and Crataegus, apomixis is in some but not all cases associated with the occurrence of more or less local and homogeneous units differing from other such units in minor ways. Intrapopulation variability is greater in sexual than in apomictic taxa in these two genera. Variability within apomictic populations, even largely sterile triploids of the Maloideae, may develop through the persistence of some sexuality, increased mutation rates, hemigamy, autosegregation, immigration, and repeated hybrid origin of apomixis. Hybridization and polyploidy are prevalent in apomictic groups of the Maloideae and contribute to their evolutionary complexity. Pollen production, which may be necessary for pseudogamy in apomicts, potentially maintains gene flow between them and sexuals or other apomictic taxa, thus retarding or preventing divergence. Neither reproductive nor monophyletic units necessarily align with phenetic units in apomicts in the Maloideae. Morphology and geography have consequently been the primary basis for species concepts in maloid apomictic groups.