A durum-sphaerococcum derivative of pentaploid hybrids

I. STANKOV¹⁾ and D TSIKOV²⁾

The specific characters of Triticum sphaerococcum PERC. have been found to be controled by a pleiotropic recessive gene s (ELLERTON 1939), or sp (MATSUMURA 1954) which appears in dominant position in T. aestivum L. SEARS (1947) determined this gene as recessive and hemizygously ineffective (null allele) and as being localized in 3D (XVI) chromosome. SCHMIDT, WEIBEL and JOHNSON (1963) assumed that a similar effect observed in T. aestivum was due to one gene that was incompletely dominant and non-allelic to the s gene in 3D chromosome. SCHMIDT and JOHNSON (1963) found a from in one population of T. durum, introduced from China, which showed characters of T. sphaerococcum and this made the authors call it T. durum ssp. sphaerococcum. Finding only 14 bivallents in it, they supposed that it was a result of a gene translocation from the D genome to the tetraploid AB genome or even that a normal gene from the A or B genome had mutated into a sphaerococcum gene. Thye assumed later that this durum form was conditioned by a gene lying outside the D genome (SCHMIDT AND JOHNSON 1966). The results obtained by BOZZINI (1965), GUPTA and SWAMINATHAN (1967), DJELEPOV and CHAVDAROV (1969), etc., with induced sphareococcoid type of mutants in T. durum and T. diococcum lend support to this assumption. Durum-sphaerococcum forms have also been found in nature and DOROFEEV (1969) considered them to be a result from spontaneous interspecific hybridization.

In hybridization of T. sphaerococcum PERC. with representative species of the tetraploid wheat group, the form-building process in the pentaploid hybrids obtained leads to a marked reduction in the frequency of occurernce of sphaerococcum type of plants. In some cases even such hybrids have not been observed (PERCIVAL 1921). Our studies on the hybrids of T. sphaerococcum with T. durum and T. dicoccum confirmed the above stated elimination of the sphaerococcum type of plants (2n=42) in these pentaploid hybrids, but we demonstrated that this type of hybrids still appear in F₂ and F₃ with frequency highly restricted (4.8 and 2.7 per cent in the crosses with T. durum, and 2.4 and 0.6 per cent in those with T. dicoccum, respectively), as in the later generations they may be preserved and stabilized through selection (STANKOV and TSIKOV 1972 ab).

In the F₂ hybrid population of the pentaploid hybrids, we also observed plants of the sphaerococcum type with traits of T. durum and T. dicoccum. Such a form with deviations from the sphaerococcum type was selected in F₅ of the T. sphaerococcum x T. dicoccum cross and its plants kept the traits characteristic of sphaerococcum till F₈ but they were of the durum type (Fig. 1). They had 28 chromosomes in the root tip cells and they were, therefore, of the tetraploid (durum) type with sphaerococcum characters, namely spherical and vitreous grain, wide and erectoid leaves, short stem, resistant to lodging, but smaller ear compared to T. durum. Meiosis in this form was normal, with 14 bivallents recorded at diakinesis.

The F₂ hybrids of sphaerococcum type with characters of T. durum and T. dicoccum observed by us, as well as the deviations found in F₅ from the sphaerococcum type towards the durum type (2n=28), which might be preserved by selection, could be a result of a trans-location of the block with s-locus of the 3D chromosome. Whether the locus with s (=sp) factor in the durum-sphaerococcum form lies outside the D genome, as assumed by SCHMIDT and JOHNSON (1966) or there is a translocation from the 3D chromosome with the block of this locus in the chromosome outside the D genome, as SCHMIDT and JOHNSON (1963), and STANKOV and TSIKOV (1972a, b) considered, or whether both situations are possible, are to be the subject of further and more precise studies. This will be significant genetic and phylogenetic importance.

(Received June 1, 1974)