GENOMIC RELATIONSHIP OF GUAYULE WITH PARTHENIUM SCHOTTII

Abstract

F 1 hybrids obtained from crosses between the rubber‐bearing species, Parthenium argentatum Gray (commonly known as guayule), and P. schottii Greenman ex Millspaugh and Chase, a nonrubber‐producing species from southern Mexico, were morphologically variable. They were generally intermediate between the two species with respect to leaf size, head size, number of disk florets per head, and the length of the peduncles. Like the parental species, the hybrids had 2 n = 36 chromosomes. They averaged 14.56 bivalents, 3.92 univalents, 0.56 trivalents, and 0.32 quadrivalents, indicating a high degree of homeology between the genomes of the two species. The observation of one quadrivalent at diakinesis in 32% of the PMCs and the frequent occurrence of a bridge accompanied by a fragment at late anaphase I of the F, hybrids suggested that the two species differ in a reciprocal translocation and a paracentric inversion. The ease of hybridization, the partial fertility of the hybrids, and their high degree of chromosome pairing indicate that P. schottii and P. argentatum are closely related in spite of their distinct morphological features and geographical distributions. This close relationship provides an opportunity to transfer genes from P. schottii to guayule for desirable characteristics, such as high biomass production and resistance to Verticillium dahliae Kleb, by means of interspecific hybridization.