Two separable stages in the process of chromosome pairing have been demonstrated. The first results in a close spatial relationship of homologs, and the second results in synapsis and formation of chiasmata. Colchicine reduces chiasma formation in conventional bivalents but not in an isochromosome. Thus, colchicine affects only the first stage of pairing.

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Greater genetic complexity has been revealed for the control of bivalency in hexaploid wheat. A suppressor of homoeologous pairing has been detected on chromosome 3A. Thus, there are two suppressors in homoeologous group 3. The 3A suppressor may be homoeoallelic to either the suppressor on 3Dβ or the promoter, detected in this study, on 3Dα. Individually these two suppressors are less effective than the suppressor on the long arm of chromosome 5B; however, their combined effect is yet to be studied. This greater complexity suggests that hexaploid wheat may not be too dissimilar to other polyploids as regards genetic control of bivalency. The mode of action of these suppressors appears to be consistent with a heteromultimeric hypothesis.

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The cereal cyst nematode,Heterodera avenue Wollenweber, is a serious pest of cereals in many countries. A high level of resistance to the unique Australian pathotype of the nematode has been demonstrated in a triticale line (T701-4-6), which was originally obtained from CIMMYT. The level of resistance is similar to that in rye cultivar, South Australian, but higher than that in the wheat line (AUS 10894), hitherto reported to have useful resistance to the Australian pathotype. The gene for resistance was located on rye chromosome 6 (6R) after backcrossing the T701-4-6 line to wheat and correlating the resistance with the presence of individual rye chromosomes identified by morphological, cytological, and isozyme markers. Preliminary evidence suggests that the gene is located on the long arm of6R. To transfer the resistance to wheat, double monosomics of6R and6D in aph1bph1b homozygous background were selected from F2 progeny from a cross of disomic6R substitution for6D to theph1b mutant. Selfed seeds from these F2 plants will be screened for wheat-rye chromosome recombinants.

Lines possessing an isochromosomc derived from the Transec translocation chromosome were isolated a~t d crossed to known telocentric 4A stocks. From observations on chromosome pairing behaviours in these crosses it was determined that the lcft arm of chromosome 4h is involvcd in the translocation. T h e ryc segment was located one crossover unit from the centromere of chron~o-some 4A by the telocentric linkage method. IntroductionFor purposes of incorporating additional sources of disease resisrances into common wheat (T~iticzmz aestivllm L.) segments of alien chromatin have been translocated to wheat chromosonles. ICnowledge as to the precise location of these translocated segments is useful in attempts to utilize such stoclts.One of these translocation stoclts, 'Transec, involves a segment of cultivated rye (Secale ce~eale L.) chromatin bearing resistances to wheat leaf rust (Pzlcci~zia reco7zdita Eriltss.) and wheat powdery mildew (E~yriphe gmnzinis tritici March) (Driscoll and Jensen, 1964). As sho~vn by monosomic analysis this rye segment is attached to wheat chromosome 4A (Driscoll and ~h d e r s o n , i967). This paper concerns the detection of the specific arm of chromosome 4A involved in the translocation and the determination of the genetic distance of the rye segment from the 4A centromere.