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If the transfer of a nucleus
into a neighboring cell is not complete, aneuploid PMCs will appear.
A loss or gain of one or more chromosomes has two obvious
possibities: firstly extremely deficient gametes will not survive and
they will be eliminated; and secondly, those gametes which contain
chromosome numbers different from the normal are able to survive. The
latter may be responsible for producing aneuploids. Fig.1
(9) shows a
70-chromosome PMC. If the transfer of a nucleus into a neighboring
cell is complete, chromosome number would double. Fig.1
(10) shows a 84-
chromosome PMC, which contains two quadrivalents (arrow a), three
ring bivalents (arrow b), three rod bivalents (arrow c) and 64
univalents. This may explain the process of polyploid formation.
Multipolar division, which occurred in this hybrid (Fig.1
(9)) might be
caused by the formation of multipolar zones of synchronized nuclei in
a coenocyte. We speculate that this kind of PMCs cannot form normal
tetrads and has to disintegrate eventually. Conversely, synchronized
nuclei in a few PMCs might form normal tetrads, following normal
bipolar division (Yen et al.1993). If this is true, the spontaneous
chromosome number doubling and redoubling might have occurred. It
might be one of the pathways of speciation in Triticeae. Such
a pathway could lead to the origin of a high level autoallopolyploid
such as L. angustus. We believe that the mechanism of
chromatin transfer through conjugation tube or opening is a kind of
variation of fertilization. According to our observation, chromatin
material migration among cells has been found in all the intergeneric
hybrids which derived from Psathyrostachys huashanica (Sun et
al. 1992b; Yen et al. 1993; and the present study). We suggest that
the N genome has a gene system for controlling this process.
References
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