Relationships among 3 Kochia species based on PCR-generated molecular sequences and molecular cytogenetics

Forage kochia (Kochia prostrata ssp. virescens 'Immigrant' is native to the arid and semiarid regions of central Eurasia. It was introduced into the United States in 1966 as PI 314929 and released as a perennial forage shrub in 1984. Kochia americana is a perennial native to the United States, whereas Kochia scorparia is an introduced annual species that became a weed. To assess both the breeding potential and the possibility of genetic contamination, relationships among the 3 Kochia species were analyzed using random amplified polymorphic DNA (RAPD) markers, sequence tagged site (STS) marker sequences of the chloroplast NADH dehydrogenase gene (ndhF), genomic in situ hybridization (GISH), and multicolor fluorescence in situ hybridization (MC-FISH). Seventy decamer random primers yielded 458 polymorphic bands from 9 plants of K. americana, 20 plants of K. prostrata, and 7 plants of K. scoparia. Fifty-four and 55 species-specific RAPD markers were identified for K. americana and K. prostrata, whereas 80 RAPD markers were specific to K. scoparia. Based on the presence or absence of informative RAPD markers, the 3 species always grouped into 3 distinct clusters in a NTSYSpc2.01b-generated dendrogram. The same relationships were found among the 3 Kochia species based on ndhF DNA sequence divergence. Using a set of 7 STS markers that can identify each Kochia species, we did not find a single interspecific hybrid from artificial hybridizations among the 3 Kochia species. In GISH studies, chromosomes of 1 species fluoresced in green only when they were probed by genomic DNA of the same species. Cross-hybridization by genomic DNA of another species was not observed. In FISH studies using pTa71 (for 18S-5.8S-26S rDNAs) and pScT7 (for 5S rDNA) as probes, there were 1, 1 and 3 pTa71 sites and 2, 1, and 1 pScT7 sites in each haplome of K. prostrata, K. americana, and K. scoparia, respectively. It is concluded that these 3 Kochia species are so genomically distinct that gene introgression among them would be extremely rare.