The Origin of Iso-Chromosomes at Meiosis.

N 1.50-CHROMOSOME is a chromosome having two identical arms A (DARLINGTON 1939). Such chromosomes have been studied recently by RHOADES (1940) in Zea and by DARLINGTON (1940) in Fritillaria. It is also very probable that the extra chromosomes in secondary trisomics, such as the numerous types which have been found in Datura (BLAKESLEE and AVERY 1938) are true iso-chromosomes. The manner in which these metacentric isochromosomes arise is of considerable cytogenetical interest. In Zea it has been clearly demonstrated that telocentric chromosomes give rise to iso-chromosomes. Furthermore it is known (UPCOTT 1937; KOLLER 1938; DARLINGTON 1939) that telocentric chromosomes result from the aberrant division-misdivision-of the centromere in univalent chromosomes a t meiosis. Consequently it has been inferred that iso-chromosomes are derived from telocentric chromosomes which themselves result from misdivision. Recently in Fritillaria DARLINGTON (1940) has observed this complete sequence of events cytologically. It is also possible that the telocentric chromosome types resulting from misdivision a t AI of meiosis may become transformed directly into functional iso-chromosomes through a rotation of their two identical arms such that the centromere assumes a median position. However, the origin of iso-chromosomes in this manner and their presence in microspore nuclei after surviving the second meiotic division have not been demonstrated cytologically. The observations to be reported in this paper have revealed a unique type of centromere behavior in normally paired and oriented bivalents which gives rise directly to iso-chromosomes a t meiosis and results in their regular presence in a considerable fraction of the resulting microspores. This anomalous centromere behavior has been found in a single diploid plant of Gasteria sp. (probably maczdata) now in the collections of the YALE DEPARTMENT OF BOTANY. It is not known where or how this particular plant originated. Chromosome behavior has been studied a t meiosis in pollen mother cells, a t the first postmeiotic mitosis in microspores, and a t somatic mitoses in anthers and root tips by means of acetocarmine and Feulgen smear preparations. The unusual behavior a t meiosis has been observed a t the time of annual flowering for two successive years and is evidently a constant character of the plant. It has not yet been possible to obtain evidence as to the manner in which the anomalous meiotic behavior is inherited, since the plant is self-sterile, and no other individuals have been available for making cross-pollinations.