THE RELATIONSHIP BETWEEN DNA CONTENT AND ALTERNATIVE MEIOTIC PATTERNS IN CERTAIN DISCOCEPHALINIDS (PENTATOMIDAE; HETEROPTERA)

Alternat ive pat terns of meiosis in a single organism provide an oppor tuni ty for direct analysis of the relat ionship between chromosome organizat ion and chromosome behavior. An obvious possibility is tha t the observed behavior ia l differences are associated with quant i ta t ive changes in the chemical consti tuents of chromosomes, par t icular ly deoxyribonucleic acid (DNA). Measu remen t of chromosomal D N A has previously been made in several forms in which normal and abnormal pat terns of chromosome behavior coexist (7, l, 2, 10). The results were uniformly nega t i ve -no difference in D N A conten t was detected between cells displaying abnorma l chromosome behavior and the more normal cells. However, these studies do not rule out relatively minor differences in D N A content nor do they exhaust the varieties of known chromosomal behavior patterns. The present study is an a t t empt to fill this gap by providing more precise D N A measurements on the unusual ly suitable and interesting al ternat ive meiotic pat terns in the discocephalinid bugs, Mecistorhinus panamensis and Neodine macraspis. The cytology of spermatogenesis in Mecistorhinus and Neodine has been described in detail by Schrader (14) and confirmed by me. The features necessary for an unders t and ing of the cytochemical da ta are as follows (based on Schrader, 14): The testes of Hete rop te ra are divided by connect ive tissue into several subunits called " lobes ." Meiosis in most He te rop te ra follows the same course in all lobes and is quite or thodox with respect to pairing, te t rad formation, and segregation bo th of autosomes and of sex chromosomes. T he unor thodox meiotic pa t te rn found in Mecistorhinus and Neodine occurs only in one l o b e t h e " h a r l e q u i n " lobe. This pa t t e rn is character ized bo th by prophase al terat ions and by changed division behavior of the autosomes. In prophase, synapsis and pachytene stages are not found. Instead leptotene is followed by progressive condensat ion of unpa i red chromosomes t h e "post leptotene condensat ion." This is followed by stages equivalent in terms of chromosome condensat ion to diplotene, confused stage, and diakinesis in normal lobes, a l though no pair ing is present. Diakinesis is character ized by the format ion of a r ing of chromosomes at the per iphery of the nucleus, often wi th the sex chromosomes (X and Y) lying inside a r ing of autosomes. Most remarkable are the meiotic divisions themselves in which the sex chromosomes show or thodox behavior , while the autosomes are markedly unusual . Thus at the first division the X and Y divide equat ional ly bu t the autosomes are aggregated into a negatively heteropycnot ic c lump which is shifted off the polar axis and from which one large autosome passes to one pole while the rest of the c lump goes to the opposite pole. At the second division the X and Y show typical " t o u c h and go" pair ing and segregate to opposite poles in bo th types of second spermatocyte. The single autosome in the smaller second spermatocyte usually passes to the same pole as the X chromosome. In the larger second spermatocyte the c lumped autosomes are dis tr ibuted irregularly to the two poles. Thus four main types of spermat id result: X plus a var iable n u m b e r of autosomes; Y plus a var iable n u m b e r of autosomes; X plus one large autosome; Y. Emphasis should be placed on the uniformity of behavior in the har lequin lobeth i s pa t t e rn shows as lit t le cell to cell var ia t ion as is found in the or thodox cells in adjacent lobes.