CYTO-GENETIC INVESTIGATIONS ON SYNTHETIC GALEOPSIS TETRAHIT

Abstract

HE general occurrence of polyploidy among the higher plants has, T especially during the last decade, led to investigations of the causes and origin of multiple chromosome numbers.These rather central problems have been attacked in several ways, among which the experimental production of polyploids is of chief interest.By various methods it has heen possible, either to cause the doubling of the same, or the summation of different genomes.The polyploid types thus obtained are -especially in cases of allo-polyploidyin many respects comparable to new species.Hence it has been concluded that polyploid species found in nature have originated in similar ways.I n later years an ever increasing number of cases of allo-polyploidy have been described and discussed.Also in the genus Galeopsis, investigated by the present writer, the formation of allo-polyploids has been studied (MUNTZING 1930 a and b).To give a short recapitulation this case was as follows.Crosses between the species G. pubescens and speciosa, both having the haploid chromosome number 8, gave highly sterile F,-hybrids.However, an F,-generation was raised, comprising about 200 individuals.One F,-plant was found to be triploid and almost completely sterile.After pollination with pollen from one of the parents, pnbrswns, the triploid plant produced one single seed.This seed gave n tetraploid plant with good fertility.This case of chromosome increase after species crossing is especially interesting as it elucidates the origin of the ))natural, tetraploid species G. Tetrrrliit L. As previously described in detail (1930 a, pp.291-93; 1930 b, pp.154-158) both the triploid F,-plant and its tetraploid daughter were morphologically undistinguishable from pure Tetrclhit.Further, the artificial tetraploid behaved like a true Tefrahit not only morphologically but also in genetical andas far as could be observed also in cytological respects (see 1930 b, p. 158).Therefore, the triploid F,-plant was called ))pseudo-Tetrahit )) and its tetraploid descen-106 ARNE MlfNTZING daiit ))AT)) (artificial Tetrnhit).On account of these and other facts (see 1930 a, p. 294) the conclusion was drawn that not olily the artificial tetraploid but probably also natural G. Tetrahit represents a synthesis o f pubescens-and speciosu-genomes.Consequently, this case might he regarded as the first instance where a species already existing in nature has been synthesised from the genomes of two other species.The primary tetraploid, the AT-plant, was used both for srlfing and for crosses with natural Tcfrahitand bifitln-lines.The progeny obtained from selfiiig of the AT-plant may be called the I,-generation of artificial Tetrnhit, or better, in order to emphasize the mode of origin, the I,-generation of sgnthetic Tetrnhit. THE I,-GENERATION,h1cthotl.s.-From the AT-plant seeds were harvested, partly from 1 rre flowering, partly from controlled selfing (double bags).Though the plant was grown in a pot a good distance apart from the other Galeopsis-cultures only seeds from the bags were used for raising the I,-generation in spring 1931.On account of the similarity of synthetic and natural Tetrnhitplants special precautions had to be undertaken in order to avoid intcrmixtures.The seeds were as usual (cf.1930a, p. 190) brought to germination in bags of moist blotting paper.The seedlings were then pricked out in boxes with sterilised soil.After about ten days the young plants were planted separately in small pots in order to get root tips for cytological purposes.When planted in the pots the plants were a11 marked with a piece of wool, which remained when later on the plants had been transplanted into the field.Here each plant was labelled separately, and the growing generation was watched daily.'I'herefore, all plants belonging to the I,-generation were with ahsolute certainty descendants from selffertilization of the AT-plant.l'lie I,-generation was raised from 276 seeds.These gave 190 seedlings of normal appearance.By overheating in a hot-bed about 60 seedlings were killed, by another accident 29 young plants were lost.Most of the remaining 101 plants were grown in the experimental gardcn.Here 7 plants died from attacks of some fungus disease which this year ravaged in the Galcopsis-cultures.The remaining 94 plants reached maturity arid were investigated with the following results.