NONRANDOM PATTERNS OF NONDISJUNCTIONAL ORIENTATION IN TRIVALENTS OF MULTIPLE ROBERTSONIAN HETEROZYGOTES OF DICHROPLUS-PRATENSIS (ACRIDIDAE)

Metaphase I orientation of centric fusion trivalents was studied in 24 single, 19 double and 3 triple heterozygotes of Dichroplus pratensis. Different populations of this South American melanopline grasshopper are polymorphic for seven Robertsonian fusions, and the polymorphisms seem to be stable. Several cytogenetic factors involved in the orienta tion and segregation of the meiotic configurations such as chromosomal length, symmetry and number and position of chiasmata, have been anal ysed in previous works. In this paper we study another factor that is relevant in the above respect in individuals with more than one hetero zygous fusion: interaction among configurations regarding orientation. Our results indicate that, when there are two or three trivalents pre sent in the MI cell, there is an interaction in such a way that the nu mber of metaphases in which the two or three trivalents are non-disjun ctionally oriented is always significantly higher than expected under a hypothesis of independence. However, the number of cells in which al l trivalents are disjunctionally oriented does not decrease significan tly, so an increase of unbalanced gametes due to this factor is not ex pected. The stability of the polymorphisms would thus not be affected.