THE LOCATION OF INTERCHANGE BREAKPOINTS IN BARLEY.: II. CHROMOSOME PAIRING AND THE INTERCROSS METHOD

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Abstract
Diallel crosses between stocks of interchanges involving the same 2 chromosomes were studied for eight 2-3, four 3-7, and five 1-7 interchanges in Hordeum vulgare L. A ring of 4 chromosomes was observed at metaphase 1 when the breakpoints of 2 crossed interchange stocks were known to be in different arms of only one chromosome and in the same arm of the other chromosome. When the breakpoints were in the same arms of both chromosomes, or in different arms of both chromosomes, only bivalents were observed. Chromosome configurations exceptional to this pattern may occur in intercrosses when one or more of the breakpoints are close to the ends of the chromosomes. These observations indicate that chromosome pairing in barley is initiated at or near the ends of the chromosomes, not at the centromeres. The exceptional configurations can readily be detected by crossing the interchange stocks with a non-interchanged stock. When bivalents are the predominant chromosome associations, the degree of spore abortion may furnish additional evidence as to break positions. Those with significantly less than 50% sterility most likely have their breakpoints in the same arms of both chromosomes. The Intercross Method of locating breakpoints of interchanges that involve the same 2 chromosomes is proposed and is particularly useful in species in which observations at pachytene are difficult or impossible. Because of the similarity of chromosome pairing in barley, Zea mays and Oenothera, the proposed Intercross Method is probably adaptable to many species.