Cytological Study and Hybridization in Two Sections of <i>Verbena</i>

Abstract

The species of Verbena studied here fall into two cytological classes, one wich a basic number n=5 chromosomes, the other wich n=7. The chromosomes of the n=7 dass were approximately half to one sixth as large as those of the n=5 dass. This classification was found to correspond to taxonomical classification. The genus is divided taxonomically into two sections: Glandularia and Verbenaca. The species of the latter seetion are further divided into two series: Pachystachyae and Leptostachyae. All the species with n=5 and n=15 chromosomes belong to the Glandularia section. The polyploid forms of the n=7 dass and V hispida, a diploid form, are classified under Pachystachyae, and the other diploid species under Leptostachyae of the Verbenaca section.Hybridization was attempted between all the species under investigation. Hybrids were obtained, first between the species with n=5 chromosomes and between the species wich n=5 and n=15 chromosomes; secondly, between any two species wich n=7 chromosomes and between the species with n=7 and n=14 chromosomes, but not between n=7 and n=21, nor between n=14 and n=21 chromosomal species. Hybrids were not obtained from crossing species of the two classes referred to above with a basic difference of n=5 and n=7 chromosomes. In all Gases the hybrid plants were intermediate in general characteristics between their parent plants.A diploid and a hexaploid species in Glandularia section were crossed resulting in a tetraploid hybrid which was highly sterile. In the section Verbenaca successful Grosses were made between a diploid and two tetraploid species. In one Gase (V. hispida×V. bonariensis) the hybrid plants were triploid while in another (V. hispida×V. litoralis) only pentaploid hybrids were obtained, a total of five plants, two of which resembled closely the female parent and three the male parent.Hybrids were obtained between V hybrida and V. erinoides each with n=5 chromosomes. The chromosomes in the former are somewhat Tonger than in the latter. These differences could be observed both in root-tip sections and meiotic preparations.Suggestions are presented as to the mode of origin of tetraploid and hexaploid species.Two types of Gell size differences are presented, in one of which increase in volume appears to be physical, in the Sense Navaschin has described, and the other genetical.