Abstract
The present communication deals with morphological and cytogenetical analysis of five species of Pisum (P. sativum, P. arvense, P. elatius, P. abyssinicum and P. jomardii). Differences in morphological characters were found to be statistically significant indicating thereby considerable interspecific variations. Metroglyph analysis showed a close relationship between P. sativum and P. arvense and between P. jomardii and P. abyssinicum. Pisum elatius occupied an intermediate position between the two groups. On the basis of intermediate position of P. elatius, and its closer association with the F1 hybrid of P. sativum×P. arvense, and some of its cytological features, a probable hybrid origin for this species is suggested.
Interspecific hybridization in various combinations was attempeted in the species of Pisum. Two crosses, P. sativum (_??_)×P. arvense (_??_) and P. arvense (_??_)×P. elatius (_??_) were successful. The cytomorphological studies and the good fertility of the F1 hybrids suggest a close relationship between the parental species.
All the five species of Pisum showed 2n=14 chromosomes. Karyotype study revealed that P. abyssinicum possesses a completely symmetrical karyotype whereas P. jomardii has an asymmetrical karyotype. Pisum sativum, P. arvense, P. elatius and the two artificially obtained interspecific hybrids showed karyotypes which are basically symmetrical but tending towards asymmetry. The general trend of karyotype evolution in the Pisum species seems to be towards asymmetry.
All the species showed regular pairing of chromosomes during meiosis. However, a lower percentage of cells in some species exhibited chromatin bridges and laggards at anaphase I or anaphase II or at both. In the two interspecific hybrids univalents and multivalents along with the bivalents have been observed. The probable reasons for their occurrence have been discussed. Pollen fertility in the five species as well as the two interspecific hybrids was reasonably high. Cytomixis was observed in one of the interspecific hybrids (Pisum sativum T. 163×P. arvense I.C. 13961).
Based on the results of present investigation and the previous reports, seven has been suggested as the basic chromosome number for Pisum.
The significance of various factors including karyotype alteration, chiasma frequency and hybridization in the evolution of Pisum has been discussed.
