The two kinds of chromosomes 10 found in races of maize clearly differ structurally and genetically. Since N10 is much more frequently encountered, it presumably, although not necessarily, is the ancestral form. What can be said about the source of the differential segment and the large heterochromatic knob which characterize abnormal chromosome 10? Not much, but certain speculations may be fruitful. The K10 knob is unique in that it has specific genetic properties not possessed by any other knob found in races of maize. Although it contains the same highly repetitive 185 bp sequence of satellite DNA in all knobs of maize (Peacock, Dennis, Rhoades and Pryor, in press) it cannot be identical to them in DNA composition. It is not a typical maize knob. The origin of the differential segment is likewise obscure but its insertion into 10L and the translocation of the piece with the K10 knob may have taken place at the same time, both being contributed by the same chromosome. Of possible significance in this connection is the statement made years ago by L. F. Randolph that one of the pachytene chromosomes of a diploid Tripsacum dactyloides from Kansas had a terminal segment similar in appearance to abnormal 10. Randolph did not describe this chromosome in detail but he was fully cognizant of the appearance of abnormal 10 so it is a reasonable inference that it had both a segment comparable to the differential segment of K10 and a large subterminal knob. Walt Galinat has isolated as supernumeraries a number of chromosomes from diploid Tripsacum dactyloides in a maize background. Possibly the Tripsacum chromosome described by Randolph has already been isolated by Galinat. If not, it should be possible to extract it and study its synaptic behavior as a supernumerary in maize sporocytes with and without abnormal 10. If it pairs with the distal end of 10L of abnormal 10 but not with normal 10 and if this Tripsacum chromosome can induce preferential segregation and neocentromeres and also possesses the W2 and Sr2 loci, then abnormal 10 could have been produced in a maize-Tripsacum hybrid by a simple translocation. There are a lot of "ifs" in the above speculation but additional studies may be highly rewarding and indeed have a direct bearing on the evolutionary history of modern maize. There is in our opinion no convincing evidence that abnormal 10 originated from a translocation involving a B chromosome nor is there any evidence that another chromosome of the A complement donated the segment differentiating K10 from N10. The above hypothesis, which predicates Tripsacum as the source of this chromatin, is simple and we find it attractive because it is experimentally testable.

M. M. Rhoades

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