The genetical and mechanical properties of the sex chromosomes - VII.Apodemus sylvaticus andA. hebridensis
1. The diploid chromosome number in the spermatogonia ofApodemus sylvaticus andA. hebridensis is 48.
2. The sex chromosomes are unequal in size; theX is the largest chromosome in the complement, theY is the smallest. TheY chromosome is smaller inA. hebridensis than inA. sylvaticus.
3. The sex bivalent shows a uniform symmetrical configuration during meiotic metaphase in both species.
4. InA. hebridensis post-reduction is obligatory, theX andY segregate at the second meiotic anaphase. InA. sylvaticus 8% of the primary spermatocytes show pre-reduction, and 92% post-reduction, of theX andY.
5. The behaviour of theX- andY-chromosomes ofA. sylvaticus during meiosis suggests that the pairing segment is composed of a distal and a proximal region, i.e. that the centromere is located interstitially and chiasmata can be formed on either side of it.
6. InA. hebridensis the behaviour of theXY bivalent suggests that the distal pairing segment is either entirely absent or is too small for chiasma to have formed, and hence segregation of theX andY is always post-reductional.
Volume 101, 2022
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