Chromosome Studies in Tulipa and some related Genera.

Abstract

The basic number of chromosomes in Tulipa is twelve and diploid, tetraploid, and hexaploid varieties and species occur. Great differences in size of chromosomes are found among the diploid species. The size of the nucleus is correlated with the size and number of the chromosomes it contains. Correlation of nuclear and somatic size only holds between tetraploid and diploid forms of the same species. Tulipa galatica Fresn. has sixteen chromosomes of which four are very small and unrepresented in the most nearly allied species. The increased number is regarded as being due to transverse fragmentation. Differences between the species exist in the form of the chromosomes and in their size relative to one another, even where no difference of number is present. Two pairs of chromosomes with sub-median attachment are found in Lilium, Fritillaria, and in most species of Tulipa. The doubtful correctness of the position of Calochortus in the Tulipeæ is emphasized by the cytological differences that distinguis it from the other genera. The chromosome numbers found are seven, nine, and ten. Well-marked satellites occur in this genus and in conjunction with differences in size and attachment enable each chromosome in the haploid set to be distinguished individually. Calochortus Vesta, a tetraploid series, has four representatives of each of the seven kinds of chromosomes. In the species (T. primulina, T. australis, T. Orphanidea) especially studied, synapsis involves the lateral pairing the leptotene stage of threads representing whole chromosomes. Later the split separating the conjugants reappears, and each conjugant becomes longitudinally split giving a tetrad consisting of four chromatids, which become separated in the first and second meiotic divisions. The forms take on by the tetrads in diakinesis -rings, crosses, etc.–are regarded as being due to the opening out alternately of the reductional and equational splits. No cytological proof of the existence of crossing over has been found. This work has been carried out whilst holding a studentship of the John Innes Horticultural Institution, and I have to record my gratitude to the late Dr. Bateson for his unfailing interest and encouragement.