MULTIVALENTS RESULTING FROM MONOBRACHIAL HOMOLOGIES WITHIN A HYBRID ZONE IN DICHROPLUS-PRATENSIS (ACRIDIDAE) - MEIOTIC ORIENTATION AND SEGREGATION

Natural populations of the South American grasshopper Dichroplus pratensis differ for seven polymorphic centric fusions involving the six longest telocentric autosomes of the karyotype (L1-L6). Hence, the same telocentric is involved in more than one fusion in different populations producing metacentrics with monobrachial homologies. The present study concerns the meiotic behaviour of quadrivalents and quinquivalents formed in natural hybrids that occur in two hybrid zones between chromosomal races with monobrachial homologies. The analysis revealed that: (a) non-alternate orientation of multivalents at Metaphase I (MI) was high, ranging from 18.5 to 64.2% of the cells in 18 hybrid males. Non-alternate Prometaphase (PMI) orientation was studied in six males (two of each type of hybrid) and in all cases values were higher than in MI, which suggests reorientation between PMI and MI. (b) All hybrids showed high frequencies of aneuploid and diploid second spermatocytes which are the result of abnormal segregation and lagging of chromosomes involved in multivalent formation. A highly significant correlation exists between the frequency of abnormal MI cells per male and abnormal second division spermatocytes. In most individuals, however, the frequency of abnormal second spermatocytes was lower than that of abnormal MI, which suggests further reorientation of multivalents towards alternate orientation at MI or spermatocyte selection between both meiotic divisions. (c) The hybrids have an increased production of macrospermatids. The behaviour of the multivalents suggests that the inter-racial hybrids have their fertility moderately to severely reduced which infers the existence of post-mating reproductive isolation between races. This is discussed in relation to the maintenance and adaptive role of the fusion polymorphisms in nature.