Segmental Interchange in <i>Pisum sativum</i>

Abstract

1. The relations between three different lines of Pisum sativum, the K line, the T line and the N lines, are discussed, with regard to chromosome structure. All three lines have seven pairs of chromosomes at meiosis. The K line when crossed with N lines was found by HAKANSSON to give a ring of four chromosomes at meiosis. Similarly the T line gives a ring of four chromosomes with N lines. The K line when crossed with the T line gives an association of 6 chromosomes at meiosis.2. This shows that the K line and the T line are related to N lines by a single interchange of chromosome segments and to each other by a double interchange, involving three chromosomes of the haploid set.3. Metaphase pairing is found to be due to chiasma formation, interstitial as well as terminal chiasmata being present at metaphase as in previously described Pisum plants.4. About 78% of the configurations of six chromosomes examined have an interstitial chiasma, the “median chiasma, ” between nonadjacent chromosomes of opposite parental complements. When this chiasma is present, the configurations assume the shape of a “figure-of-eight, ” similar to that described in Oenothera by DARLINGTON (1931).5. The presence of the “median chiasma” shows that an homologous interstitial segment is present in the otherwise non-homologous chromosomes taking part in it. A hypothesis is put forward to explain the origin of this segment.6. The “figure-of-eight” is shown to be a cytological demonstration of previous genetical crossing-over, on the single assumption that only homologous parts of chromosomes pair. This crossing-over gives segmental interchange between chromosomes only homologous in an interstitial region.7. As a result of this segmental interchange, there are four viable gametic types containing n-chromosomes possible, namely the two parental types and two new types containing the interchanged chromosomes. The different possible zygotic combinations to be obtained on selfing this plant are predicted. Instead of the progeny being restricted to plants with simple pairing and plants with associations of six chromosomes at meiosis, plants with rings of four chromosomes are expected, as a result of the new combinations resulting from segmental interchange.8. A re-examination of the ring of four chromosomes formed when the T line is crossed with N lines showed the presence of chiasmata on both sides of the attachment constriction in one pair of segments, in some cases. This demonstrates that the point of attachment is at some distance from the place where the change of segment occurs. It is suggested that such configurations occur also in Zea Mays and in Datura.The author desires to thank Miss C. PELLEW for supplying the material for cytological study, Dr. C. D. DARLINGTON and Dr. F. W. SANSOME for criticism and advice, and Mr. H. C. OSTERSTOCK for taking the photomicrographs and for assistance with the diagrams.