CHROMOSOME STUDIES IN ARABIDOPSIS THALIANAl

HE advantages of using Arabidopsis, a small crucifer, for studies of mutation Tand other genetic problems were pointed out by G. P. R ~ D E I (1962). His establishment of five linkage groups ( R~~DEI , private communication) and the occurrence of certain aberrant mutants made a study of Arabidopsis chromosomes desirable. The following is a report on the cytology of normal Arabidopsis thaliana (L.) Heynh. and the five primary trisomics which have been obtained. As early as 1907 LAIBACH gave the chromosome number of A . t k l i a n a (Stenophragma Thalianum) correctly as 2n=lO. His determination was based on meiosis and on prochromosomes (chromocenters) of the nondividing nucleus. LAIBACH worked with a variety of plants and was mostly concerned with proving the individuality and continuity of chromosomes, still a controversial question at that time. WINCE (1925) published a camera lucida drawing of a second meiotic metaphase showing the two plates of five very contracted chromosomes. At this stage. depending on the view, the chromosomes are almost spherical or dumbbell shaped, and no individual identification is possible. JARETZKY (1928) confirmed the presence of five pairs of chromosomes from a second meiotic metaphase plate. MANTON ( 1932) determined somatic chromosome numbers of A. pumila (2n=32) and A . suecica (2n=26 to 28) . She states “The results of other investigators with A. thaliana (n=5) are exceedingly anomalous and, in spite of the number of records, should be confirmed by a somatic count.” TITOVA (1935) from somatic counts gave the number as 2n=6. Her drawing suggests that this was some other species. It seems very unlikely that A . thaliana could occur with three pairs of chromosomes. On the other hand, close somatic association might lead to misinterpretation; but in her drawings the two satellited chromosomes lie far apart and this would make it unlikely, according to the author’s experience, that the chromosomes showed somatic pairing. The past major difficulties of cytological determinations and the absence in the literature of photomicrographs of the chromosomes of Arabidopsis can be attributed to their small size both in meiosis and mitosis. The anthers within a flower are not well synchronized, and one anther often contains no more than half a dozen cells at a useful stage of meiosis. Although mitoses are frequent, the chromosomes are tiny and may be obscured by free floating prochromosomes from broken cells. It is possible to utilize the prominent prochromosomes for somatic counts, as has indeed been done by ROBBELEN (1957), since these