abstracts of papers presented at the 1962 meetings - Genetics

ABSTRACTS 993
body” is observed in meiosis of 19 species of Bromus, representing five sections of this genus. The
nuclear body is first visible at pachytene, and remains visible for periods ranging from metaphase
I to the quartet, the duration varying from species to species. It decreases in size during later
stages of meiosis. The nuclear body is included in the prometaphase clump, is excluded from the
spindle, and is generally located near the pole at metpahase I; it does not divide, and is included
in only one cell of the dyad and one cell of the quartet. This structure varies markedly in size
among species of Bromus; there is some relationship, though not a precise one, between its size
and the chromosome number. The nuclear body appears to consist of numerous small, thin-
walled globules or of relatively few thick-walled globules, or a combination of the two. The
nuclear body is not associated with the nucleolus, nor with the nucleolar organizing regions of
chromosomes. It is sometimes in contact with a bivalent at pachytene, and this may involve a
terminal, subterminal or interstitial region, and sometimes bivalents with different chromomere
patterns. The nuclear body is more often in contact with bivalents in some species than in
others; thus the contacts may not be entirely fortuitous. The nuclear body has not to date been
observed in mitotic cells. Its chemical constitution is now being investigated.
WEILER, CONRAD P., City of Hope Medical Center, Duarte, Calif.: Sex chromosome behavior
of amphibians. I. Ram pipiens-Genetic studies and breeding experiments on fishes, amphibians,
and reptiles have shown the sex-determining mechanism to be either the XY-XX type
(male heterogamety) or the ZZ-ZW type (female heterogamety). Studies also showed many
similarities in genetic constitution between the X and the Y, or the Z and the W. Cytological
studies reported so far support the relatively undifferentiated state of the sex chromosomes of
lower vertebrates.-In order to elucidate the mechanism which prevents extensive crossing-over
between the female-determining X and the male-determining Y during meiosis of the heterogametic
sex, mitotic and meiotic figures of the leopard frog were studied.-Alignments of
metaphase chromosomes from Rana pipiens show that both sexes have 13 identical pairs; thus
the X and Y are morphologically identical. During spermatogenesis, no particular pair of
chromosomes exhibits positive heteropycnosis as does the mammalian sex pair, but chiasma frequency
for all bivalents is definitely lower in males than in females. Since isolation of factors
determining opposing sexes is crucial to the preservation of a genetic sex-determining mechanism,
it is proposed that in Rana pipiens the isolation is accomplished by the lower chiasma rate observed
during meiosis in the heterogametic male. (This work supported in part by PHS grant
CRT-5089.)
WEIR, J. A., and G. SCHLAGER,
University of Kansas, Lawrence, Kansas: Selection for total
leucocyte count in the house mouse.-A high leucocyte line (LCH) and a low line (LCL) were
selected from an outbred strain designated T. Selection was continuous for 25 generations and
was based on family and individual merit coupled with intensive inbreeding (mostly full-sib
matings). Selection was effective from the onset with a maximum difference of over 100 percent
of the original leucocyte number by generation 13 (LCH 17,500, LCL 4,000 cells per nun3).-
Although the selection differential was uniform in the two lines, the intensity of response was
greater in the LCH with the initial number of leucocytes doubled by generation 13. Response
in the LCL was more gradual but continued until the end of the experiment. The asymmetry
of response was statistically significant. Leucocyte counts in the T strain corresponding to gen-
erations 2, 5, 7, IO, and 16 remained relatively constant to generation 10 (mean 8590) but
dropped to 5105 in generation 16. The downward trend in the three lines after generation 13
may be due to environmental factors. Ninety-five percent confidence limits set to the estimate of
realized heritability for the first 13 generations were 14.7 and 22.7 percent. The selected lines
did not differ significantly in two components of fitness measured: numbers of progeny born or
weaned, and weaning weight.-Leucocyte numbers confer resistance to irradiation, but not to
artificial infection with mouse typhoid. The LCH and LCL strains were both more susceptible to