THE NUCLEAR CYTOLOGY OF BIVALENT AND RING-FORMING RHOEOS AND THEIR HYBRIDS

The structural hybridity of the chromosomes of Rhoeo spathacea (R. discolor) that produces a ring of 12 chromosomes at meiosis is probably maintained by a balanced lethal system active in the zygotic or embryonic stage. Bivalent-forming plants have been identified that belong to the taxon R. spathacea var. concolor. In the clone studied the arm arrangement in R. s. var. concolor is very nearly the same as the arm arrangement in one of the balanced translocation complexes of R. spathacea. STEARN (1957) has pointed out that the plant commonly referred to as Rhoeo discolor (L'Heritier) Hance was described first by Olof Swartz under the name Tradescantia spathacea. The rules of botanical nomenclature dictate that the correct name for this species is Rhoeo spathacea (Swartz) Steam. The typical form of the taxon has leaves that are purple beneath; it is commonly cultivated as an ornamental plant. There is an uncommon form with green leaves which was described by Baker as var. concolor. Rhoeo spathacea probably was originally endemic to Nicaragua and British Honduras. It seems to have been introduced into Jamaica and Trinidad and promptly became naturalized (Simmonds, 1945; Stearn, 1957). By now it may be naturalized over a wide area outside its original range. Cytologists have long regarded R. spathacea as a curiosity because it displays a ring of 12 chromosomes at meiosis, translocations having occurred among the chromosome arms to bring about this structural heterozygosity. No bivalent formers have ever been reported. In order to preserve this structural hybridity in the progeny of such plants, some sort of lethal system must function to eliminate the homozygotes. It is the purpose of this paper to report on the general type of lethality that functions and to note that R. spathacea var. concolor is not a structural hybrid. Var. concolor carries in the homozygous condition one of the translocation complexes present in the ring-forming R. spathacea. MATERIALS AND METHODS-Plants of Rhoeo spathacea (Swartz) Steam and R. spathacea var. concolor Baker were obtained from Oakhurst Gardens in Arcadia, Calif. Mr. J. Gridilian, I Received for publication 17 October 1967. This investigation was supported by PHS Grant No. GM 11702. The excellent technical assistance of Mary Winnike and Regina Ross is acknowledged. proprietor of Oakhurst Gardens, indicated that he had collected the plants of R. sp. var. concolor in low-lying hills of British Honduras where it was found growing in cracks of rocks in shaded areas and was mixed indiscriminately with the purple-leaved R. spathacea. Only two clones of var. concolor were available from Mr. Gridilian and these had narrower leaves and seemed somewhat less robust than the 14 clones of R. spathacea that are growing in our greenhouses. At their widest the leaves of R. sp. var. concolor were 3.0-3.5 cm and R. spathacea, 4.5-5 cm. The leaf length was usually similar and the inflorescences were similar. Our plants of R. spathacea were of diverse origin, either field-collected in British Honduras or garden plants of Costa Rica, Trinidad and Great Britain. Pollen mother-cell squashes were prepared of metaphase I of meiosis. They were either stained by the Feulgen method after fixation in 3 parts ethyl alcohol: 1 part acetic acid, or fresh anthers were squashed in acetocarmine. Slides were made permanent by the dry-ice technique. In estimating pollen viability it was assumed that viable grains stained with aniline blue in lactophenol (Darlington and LaCour, 1960). Reciprocal pollinations were made on one clone of R. spathacea and one of R. sp. var. concolor. Nearly mature buds were emasculated and immediately cross-pollinated with pollen taken from open flowers. RESULTs-The several clones of R. spathacea growing in our greenhouse all showed the typical ring of 12 chromosomes at metaphase I of meiosis. The two clones of R. sp. var.'concolor clearly showed six bivalents at metaphase I. Seeds were obtained from R. spathacea 9 X R. sp. var. concolor, but no seeds were obtained of the reciprocal cross. Well over 100 pollinations were made with var. concolor as the pistillate parent and several modifications of the pollinating procedure were used with no seed set.