CYTOGENETIC STUDIES IN AMARANTHUS: II. NATURAL INTERSPECIFIC HYBRIDIZATION BETWEEN AMARANTHUS DUBIUS AND A. SPINOSUS

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Abstract
Spontaneous triploid hybrids (2n = 49) were discovered between Amaranthus spinosus L. (2n = 34), a diploid species, and A. dubius Mart. ex Thellung (2n = 64) which was found to be a tetraploid. Meiosis in the hybrids was irregular and 15 univalents were most frequently found along with the bivalents pairing apparently allosyndetically (15 I's + 17 II's = 2n = 49) at metaphase I. Univalents were excluded from the telophase nuclei in both meiotic divisions resulting in supernumerary microspores and in a reduction in the mean size of the microspores. Consequently, the triploids were largely sterile and the few undersized seeds produced failed to germinate. Seed weight, seed volume, stomatal size, and pollen grain size were proportional to chromosome number in the parents only, not in their hybrids. Since A. dubius exhibits typical bivalent behavior in synapsis, it is considered to be an allotetraploid in which A. spinosus has been one progenitor. It is suggested that the diploid A. quitensis H. B. K. (2n = 32) might be the other progenitor, but from chromosome number relationships and morphological considerations more than two species may be involved. As a result of the high sterility of the triploids, gene exchange between the parental species must be of a very limited nature.

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