A CYTOLOGICAL SURVEY OFZEA-TRIPSACUMHYBRIDS

Abstract

Reciprocal crosses involving various combinations of diploid (2n = 20) and tetraploid (2n = 40) Zea mays L. (maize), diploid (2n = 20) Zea mexicana (Schrad.) Kuntze (annual teosinte), and tetraploid (2n = 40) Zea perennis (Hitchc.) Reeves and Mangelsdorf (perennial teosinte) were studied cytologically. Observations of chromosome behavior during meiosis indicated that chromosomes of Z. perennis were more compatible with those of Zea mays than with those of Z. mexicana. Hybrids between Z. mays and diploid (2n = 36) or tetraploid (2n = 72) Tripsacum dactyloides (L.) L, incorporating different numbers of genomes from each parent and ranging in chromosome number from 2n = 28 28 to 2n = 82, were also examined at meiosis. Allodiploids (2n = 28) combining one Zea genome with one Tripsacum genome showed a low level of synaptic bivalent formation, indicating limited chromosomal homology between the two genera. Almost without exception, maize-Tripsacum hybrids exhibited more chromosomal association than expected on the basis of strictly homologous pairing; however, only a few plants possessed configurations stable enough to be taken as evidence of possible intergeneric segmental exchange. Triple hybrids involving maize, teosinte, and Tripsacum were produced in several combinations by pollinating maize-Tripsacum hybrids with annual or perennial teosinte. These showed chromosomal associations in accordance with the expected, on the basis of cytological data from maize-Tripsacum and maize-teosinte hybrids. These cytological observations suggest that teosinte and maize are conspecific, and that teosinte could not have originated as a by-product of Zea-Tripsacum introgression.