Agropyron trachycaulum (Link) Malte and Agropyron spicatum Pursh, both native range grasses of the Great Basin, often hybridize in nature. A. trachycaulum, is a tetraploid with 28 chromosomes. It has awnless or awntipped lemmas and small anthers. A. spicatum is a diplois with 14 chromosomes. It is characterized by having long awn that bend at maturity, and anthers which are about four times longer than A. trachycaulum. Artificial hybrids are intermediate between parents in awn and anther lengths and have 21 chromosomes. A natural population in Utah was found consisting of A. trachycaulum, A. spicatum, plants resembling the artificial hybrids, and intermediates exhibiting the whole spectrum of variation between the two parents. Regardless of the morphological variation expressed by the natural population only three levels of chromosome numbers are found. Plants have either 14, 21, or 28 chromosomes. This means that only gametes on the seven or fourteen chromosome levels are functional. Only five seeds from 28,870 florets were harvested from the natural hybrids from Thistle Flat. Since these natural hybrids are highly sterile, interbreeding among the hybrids is probably rare. Because progeny from 21-chromosome plants have only 14 chromosomes, A. spicatum was probably the male parent. The variable morphology of A. trachycaulum accompanied by reduced pollen and seed fertility suggests that it has been introgressed with A. spicatum characters. Therefore, both cytological and morophological evidence indicates reciprocal introgression between A. trachycaulum and A. spicatum. Even though A. trachycaulum and A. spicatum have different levels of chromosome numbers, this is not a strong enough genetic barrier to prevent genes to flow between them. Because of introgression considerable variation can be maintained which may be of considerable evolutionary importance to both species.



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Plant and Wildlife Sciences



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