Pairing and transmission of a small accessory iso-chromosome in rye

Summary and conclusions

A small derivative of the standard type of B chromosome in rye was definitely demonstrated to be an iso-chromosome, (i) because both arms have the same chromomere structure, (ii) because the two arms of the same chromosome frequently pair with each other and (iii) because the four arms of a bivalent at pachytene may exchange partners with each other, forming a pseudo-tetravalent.

The small iso-fragment is derived from the standard fragment because the arms have a chromomere structure identical to that of the short arm of the standard fragment. This identity implies (i) that the three main chromosome regions of the short arm of the standard fragment (cf.Lima-de-Faria 1952) are also present in both arms of the small iso-fragment, (ii) that the size and distribution of the chromomeres is the same and (iii) that the cone shaped terminal part of the short arm constituted by three chromomeres may be distinguished in both the mother chromosome and its derivative, the small iso-fragment.

Owing to a high degree of meiotic elimination the frequency of transmission of the small iso-fragment is low even in plants with 2 such chromosomes.