MEIOTIC IRREGULARITY IN SPECIES AND INTERSPECIFIC HYBRIDS OF PINUS

This paper describes a comparative analysis of meiotic conditions in 61 individual trees representing 21 species and 22 interspecific hybrid combinations of the genus Pinus. Material was collected during three successive growing seasons at the Eddy Arboretum of the Institute of Forest Genetics at Placerville, California. Meiotic irregularity occurred in all species and hybrids examined; mean irregularity frequencies of individual trees ranged from 0 to 47.2 percent. Abnormalities in chromosome movement and their consequences, (1) precocious disjunction associated with the occurrence of univalents and (2) the failure of chiasma terminalization leading to lagging chromosomes and to chromosome breakage and fragments, account for most of the observed irregularity. The same kinds of irregularity occur both in the species and in the hybrids, but they were considerably more frequent in certain of the hybrids than in the related species. These abnormalities in chromosome movement seem to be characteristic of Pinus and are attributed primarily to rrechanical difficulties which attend the large pine chromosomes in meiosis. The most spectacular meiotic irregularities were the characteristic bridge-fragment configurations considered to be the result of crossing-over in heterozygous paracentric inversions. Inversion bridges were observed in 59 of the 61 trees and were as frequent in the species as in the hybrids. They apparently do not result from interspecific differentiation in chromosome structure but from spontaneous intra-specific rearrangements. The literature and work now in progress provide increasing evidence that introgression has been an important factor in the evolution of pine populations. The cytological study of pine chromosomes has failed to produce qualitative evidence of introgression, but the quantitative measurement of meiotic irregularity may serve as a useful criterion for recognizing the results of past hybridization.