Identical linear order of chromosomes in both gametes of the acoel turbellarian Polychoerus carmelensis: a preliminary note.

In an exceptionally well-defined first cleavage metaphase in a sectioned egg of Polychoerus carmelensis all 34 (2n) chromosomes were present in a single 8-micron section, and it was possible to identify the homologous chromosomes derived from the two parent gametes. Three groups of chromosomes from one parent, containing 7, 7, and 3 chromosomes, respectively, could be exactly matched by corresponding groups of homologues derived from the other gamete. The probability of such an ordered pattern occurring by chance is somewhat less than 1 in 2(48). The simplest explanation of this arrangement is that the 17 chromosomes contributed by the egg were in precisely the same linear order as those contributed by the spermatozoon. This suggests that during certain stages preceding first cleavage metaphase, the 17 members of each haploid set of chromosomes may have been attached, end-to-end, in a linear fashion, by highly specific bonds.