Results of two F1 from RILEY and KIMBER (1966) (Table
2) allow to compare parallel hybrids including T. aestivum and,
alternatively, Ae. speltoides or Aegilops longissima SCHWEINF.
and MUSCH. (2n=14). The mean number of bivalents of the F1 (T.
aestivum x Ae. speltoides) is practically the same as in the
present hybrid. Taking into account the different chromosome number, and
substracting from the total frequency (6.77%) the heteromorphic associations
implying the Agropyron telosome, the corresponding figure is 6.65.
Higher associations give an overall frequency of pairing about 5% higher
in this case. It can be stated that the presence of the alien chromosome
has no disturbed free homoeologous pairing involving Triticum and
Aegilops genomes. DVORAK data (1972) belong to two F1
between a ditelosomic addition line (Agrus Tc7) and
Ae. speltoides with a "low" (I) and a "high" (III) levels of activity.
Even in spite of a higher bivalent frequency, the mean number and the %
of associated chromosomes are lower than those of RILEY and KIMBER and present
data. DVORAK and KNOTT (1972) also reported that with Ae. speltoides
(level III), the % of heteromorphic associations involving the Agropyron
telosome (7 Ag) was 12.3 against respectively 45.0 and 77.0 for the wheat
telosomes 7B or 7A. DVORAK (1971) reported pairing of three other A.
elongatum telosomes with wheat chromosomes but frequencies are not indicated.
It is interesting to note that for another A. elongatum telosome
(homoeologous with group 6) JOHNSON and KIMBER (1967) found that it was
involved in heteromorphic associations in an overall frequency of 4.8% (0.0
to 10.8) of PMC, whilst, 18 wheat telosomes tested exhibited a mean pairing
of 51.1% (34.0 to 71.3). In this work, it must be emphasized that the pairing
frequency of 15% observed for the Agropyron telosome was recorded
in a higher overall pairing frequency background (Table
2), as compared with the other mentioned cases, but there is no wheat
telosomes behaviour for comparison. In turn, JOHNSON and KIMBER (1967) made
no estimation of overall frequency.
It is evident that Agropyron telosomes pair much less frequently
than any of the wheat telosomes observed. Some explanation of this contradiction
observed between the pairing behaviour of the Agropyron telosomes
and their ability to substitute very efiiciently for wheat chromosomes,
was advanced by RILEY and CHAPMAN (1966). These authors think that the pairing
affinities have diverged further than the factors controlling genetic equivalence,
possibly because these changes in limited and specific chromosome regions
are evolutionary more tolerated than changes with more radical effects in
vital physiological processes. Inversely, according to ATHWAL and KIMBER
(1972), two homoeologous chromosomes may have equivalence in pairing segments
while showing different alleles on them. Pairing affinities would be determined
by the interaction of two factors: the homology in nucleotide sequences
in the sites of recognition and its activation by a balanced system of genes
(DVORAK 1972). It could be reminded that, besides these factors, situation
in the particular case of telosomes might be complicated because of the
absence of one entire arm which could carry some important sites responsible
for pairing ability for the whole chromosome; this fact would be also illustrated
by the variation found in pairing frequency of the wheat telosomes. |