Chromosome Ring Formation in Rhoeo discolor

BELLING'S (1927) suggestion that chromosome ring formation in Oenothera is caused by segmental interchange between non-homologous chromosomes has received strong support during the past few years.This theory applied to the situation found in Datura and Oenothera by BLAKESLEE and CLELAND seems to meet all requirements, and since predictions of chromosome behavior in Oenothera hybrids are in accord with the theory there seems to be little doubt of its validity.According to the theory of segmental interchange no two chromo somes in a closed ring or complete chain should be identical and at least half of them are composed of segments from two non-homologous chromosomes.Any two adjacent chromosomes may be morphologically different and must be different in their genetic constitution.The order of the individual chromosomes in the ring or chain should always be the same.The chromosomes are so similar in Oenothera that the indi vidual chromosomes cannot be identified, although their consistent order in the ring is indicated by the genetic investigations.In Rhoeo discolor the twelve somatic chromosomes differ consider ably in length and position of the spindle fiber attachment and it should be possible to determine the sequence of the chromosomes in the chain at meiosis.BELLING (1927) described ring formation in Rhoeo but did not make a detailed study of chromosome size or behavior.DARLING TON (1929) made a more complete study of Rhoeo chromosomes and found four somatic chromosomes with subterminal constrictions.At metaphase of the first meiotic division he found the chromosomes in rings, in one chain, two chains or in some cases three chains.Various types of non-disjunction were described.In none of his dozen figures of metaphase chromosomes is it possible to identify all of the four heterobrachial chromosomes although one or two can usually be recog nized.KATO (1930) also has made an extensive study of Rhoeo chromo