Meiotic Pairing and Chiasma Frequency in an Induced Trisomic of Grass Pea (Lathyrus sativus L.)

Abstract

Meiotic pairing association and chiasma frequency were studied in a gamma-ray induced primary trisomic of grass pea (Lathyrus sativus L.). The trisomic was identified by phenotypic abnormalities, early maturity and presence of an extra chromosome (2n+1=15), while occurrence of 7 bivalents was the usual feature in normal diploids (2n=14). In the trisomic, formation of a univalent was more common than that of a trivalent, although bivalent association was predominant. Ring bivalents appeared more frequently than chain or rod configurations. Terminalization of 2 chiasmata in the 2 arms may give rise to ring bivalent, while a single chiasma may terminalize to form a chain. In the trisomic, frequent occurrence of linear or frying pan configuration and occasionally Y-shaped trivalent formation indicated unmodified nature of the extra chromosome; this characterized it as a primary trisomic. Terminalization of chiasmata in both the arms of a chromosome involving its 2 homologous partners may result in linear configuration, while a chiasma in the distal end with another chiasma in the same arm may give rise to frying pan configuration, but an interstitial chiasma accompanied with another chiasma formed at the terminal position distal to former might form Y-shaped trivalent. Configuration of trivalent may thus be influenced by the number and position of chiasmata. Compared to normal diploids chiasma frequency reduced in the trisomic. More frequent occurrence of unequal anaphasic separation of chromosomes may be attributed to reduced pollen fertility in the trisomic.