Abnormal pollen grains occur regularly in small percentages in nearly all anthers and may vary from one individual to another (heterogeneity of pollen forms). A much higher percentage of such deviating, more precisely, malformed pollen grains are found in many cultivated plants (ornamental plants, agricultural crops) or in plants with asexual reproduction (autogamic plants, apomicts). The effect also depends on the ploidy level, hybrids may produce a series of pollen types. Pollen variation is generally underreported, because most studies focus on normal rather than abnormal pollen morphology (BANKS et al. 2007).
Aberrant pollen may occur at a high percentage within a single pollen sac, anther or flower.
These aberrant, deformed pollen grains differ from normal pollen in shape and dimension, in number and form of apertures, in type and arrangement of ornamentation. Very often in hybrids, pollen ornamentation is intermediate between the typical forms of the parent species. Some species produce only malformed pollen, a feature typical for plants with asexual reproduction (e.g., some Alchemilla spp.).
The reasons for the production of deviating pollen forms are genetically, chemically or environmentally induced. While many reports dealing with the developmental causes, a tiny minority of papers refers predominantly to the habit of the deviating pollen forms (e.g., POZHIDAEV 2000a, b, focusing on the aperture patterning). As a consequence, these outliers should neither be ignored nor overrated, but we should take notice of this issue as an ordinary feature.
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