A CYTOGENETIC STUDY OF INVERSIONS IN ZEA MAYS

Abstract

LTHOUGH spontaneous and induced inversions have been reported in A many organisms, the genetic behavior of inversions is known exclusively from studies of Drosophila. It is the purpose of this study to investigate inversions in Zea mays, a t present the only organism other than Drosophila in which a relatively precise genetic and cytological analysis could be made. MULLER (1940) proposed that inversions including the centromere be called pericentric and inversions limited to a single chromosome arm be designated as paracentric. Heterozygous inversions of both types form loop bivalents at meiotic pachytene or in the salivary gland chromosomes due to the pairing of the inverted and normally arranged homologues. Single crossover chromatids arising from exchanges within the limits of a heterozygous paracentric inversion are dicentric and form a chromatin bridge a t meiotic anaphase or are acentric and lost because of their failure to undergo regular mitotic behavior (MCCLINTOCK 1933). The dicentric and acentric crossover chromatids carry genetic duplications and deficiencies. Single crossover chromatids arising from exchanges within heterozygous pericentric inversions carry genetic duplications and deficiencies but such chromatids each have a single centromere and no chromatin bridges are produced. Double crossover chromatids arising from multiple exchanges within either heterozygous pericentric or paracentric inversions do not carry duplications and deficiencies and may be recovered in viable offspring. Duplication-deficiency chromatids ordinarily cause abortion of the gametophyte generation of plants. In animals duplication-deficiency chromatids result in aneuploid gametes which, though functional, give rise to zygotic or embryonic lethals. Consequently, heterozygous inversions have been found to reduce greatly genetic recombination within the inversion segments in Drosophila melanogaster (STURTEVANT 1926; 1931 ; STURTEVANT and BEADLE 1936) and in D. pseudoobscura (DOBZHANSKY and EPLING 1948). In the inversion heterozygote, crossing over is also frequently reduced in the regions adjacent to the breakage points (STURTEVANT and BEADLE 1936; DOBZHANSKY and EPLING 1948). Pericentric inversions are not abundant in natural populations, due apparently to the mortality caused by aneuploid gametes. Two spontaneous peri-centric inversions have been reported in Drosophila robusta (CARSON and STALKER 1947) and one in D. algonquin (MILLER 1939). In contrast to the scarcity of spontaneous pericentric inversions, there is an abundance of para-Submitted in partial fulfillment for the degree of DOCTOR OF PHILOSOPHY in the