Identification of an isochromosome for the long arm of chromosome 7 in maize
--Muzumdar, DA; Schneerman, MC; Doyle, GG; Weber, DF

Doyle (1988, MNL 62:49-50) analyzed large numbers of selfed progeny of maize primary trisomics and identified "presumptive telocentrics" for several chromosome arms on the basis of deviant genetic ratios. Numerous crosses involving two presumptive telocentrics for 7L have been analyzed by DW and MS, and one of these (telocentric 7La) gave genetic ratios that were consistent with the presence of a telocentric for 7L. Pachytene in the original genetic background was examined but found to be unsuitable. This telocentric (and others) was crossed with KYS, an inbred with exceptionally favorable meiotic cytology. Pachytene cells in these revealed that the extra chromosome had two arms, and work with it was temporarily abandoned. Recently, we (DM, MS, and DW) re-examined it and recognized that the additional chromosome was an isochromosome. It is interesting to note that of the 5 presumptive telocentrics that were recovered by Doyle and cytologically confirmed, there were two independently isolated isochromosomes for 4S, two independently isolated telocentrics for 6L, and an isochromosome for 7L. In addition to these, telocentrics for both arms of chromosome 3 were recovered by Rose and Staub (MNL 64:94-95, 1990), and we have examined these extensively and confirmed that these are indeed telocentric for both arms of this chromosome. Rhoades (1933, PNAS 19:1031-1038 1936, Genetics 21:491-502; 1938, Genetics 23:163-164; 1940, Genetics 25:163-164) previously recovered and extensively studied a telocentric for the short arm of chromosome 5 (5S) in maize from a trisomic-5 plant. He (Rhoades 1933, 1940) found that the telocentric-5S gave rise to an isochromosome with a low but consistent frequency. The origin of this chromosome was hypothesized to be a transverse misdivision of the centromere during a pollen grain mitosis leaving the two arms attached. Unfortunately, this telocentric and isochromosomes derived from it have been lost (E. Dempsey, Indiana Univ., personal communication). It is not known if the isochromosomes for 4S and 7L originated as telocentrics which subsequently gave rise to isochromosomes or if they originated as isochromosomes. These telocentrics and others which may be recovered are powerful tools for analyzing the maize genome. 

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