| Besides some obvious differences in experimental conditions, general differences
in pairing frequencies, as those shown in Table 2,
may well be caused by some other factors. In one complex interspecific hybrid
(each one is a unique individual recombination product) there must be, within
the limits of its homoeology, a competition in pairing ability between participating
chromosomes, in this case: Triticum, Aegilops and the Agropyron
telosome. Three are few quadrivalents and higher associations in 5B-deficient
haploids of T. aestivum but these associations are more frequent
when Aegilops sp. are included in 5B-deficient hybrids (RILEY and
KIMBER 1966); pairing between wheat and Aegilops chromosomcs is shown
when Ae. speltoides or Aegilops mutica BOISS. (2n=14) are
used to suppress the effect of 5B (RILEY and CHAPMAN 1964). Indeed, overall
pairing frequency obtained here (24.23%) correspond to 16.71 if only 20
chromosomes were present and is substantially higher than the reported 12.07
in nulli-5B haploids of T. aestivum (KIMBER and RILEY 1963). According
to previous work in this field and to present data it is possible to state
that it would not be easy to employ this method as a simple additional tool
in conventional plant breeding. There are two main handicaps: difficult
interspecific hybridization and subsequent hybrid sterility. This difficulty
to get any seeds from this type of cross, is shown by present data and unpublished
results obtained in this laboratory both under greenhouse and field conditions.
Even if this problem could be presumably surmounted by means of embryo culture,
it would still remain another major obstacle in the weak viability of resulting
seeds and in the high sterility of the first generations. It is possible,
however, to get new recombinations through this method; they will give,
after the first sterile generations, enough individuals to allow further
selection. It could be useful to remark the importance of genotypic variability of the different species employed. In the case of Aegilops, this was pointed out by FELDMAN and MELLO-SAMPAYO (1967). Because Ae. speltoides is a cross-pollinated species a high heterozygosity is expected for many characters, including homoeologous pairing promotion in hybrids; this is still complicated by its spontaneous interspecific crossability within the genus. It is not surprising then to find such differences, as reported for promoting levels (DVORAK 1972), for Ae. mutica pairing induction (RILEY et al. 1971) and for both Ae. speltoides and Ae. mutica homoeologous pairing promotion (DOVER and RILEY 1971). Moreover, intraspecific variability present in the Agropyron source employed may determine some differences in pairing behaviour and it must also considered the genetic back-ground of T. aestivum which has been crossed: e g, TAF 2d has been derived from a "V. 27" back-ground and structural chromosome differences between this cultivar and Chinese Spring are known (THE and BAKER 1970, CAUDERON 1974). Finally, differential interactions between other identified genetic factors responsible for normal meiotic pairing and genetic systems involved in Ae. speltoide (or mutica) (FELDMAN 1968) may also be responsible of differences in homoeologous pairing in interspecific hybrids. (Received June 1, 1974) |
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