The DNA content of spermatozoa from different strains of mice.

Abstract

Certain morphological characters of mammalian spermatozoa have been shown to have a high heritability (Beatty, 1970). Sperm dimensions may be altered by genetic selection (Woolley, 1970). In addition to the biological action ofthe genes, the actual mass of the DNA and proteins, which constitute the major components of the head, may affect the sperm phenotype. Unlike genetically active cells in which nuclear size is governed, to a certain extent, by metabolic factors, the sperm genome is condensed, homogeneous and inactive. It might therefore be expected that head size would be influenced both by the quantity of DNA present and by the way it is condensed. Thus diploid spermatozoa have enlarged heads (Beatty & Fechheimer, 1972), whereas spermatozoa with abnormally small heads might have a lower Feulgen\p=m-\DNAcontent than morphologically normal members of the same ejaculate (Gledhill, 1970). The size differences described in the latter studies probably reflect alterations in sperm chromatin structure affecting the availability of Feulgen-reactive sites. When the DNA contents of small and normal spermatozoa were compared by ultraviolet (u.v.) absorption methods, no significant differences were observed (Gledhill, 1970). There are also many instances in the literature of variations in Feulgen\p=m- and u.v.-DNA contents in morphologically normal spermatozoa of fertile and subfertile animals (Leuchtenberger, Leuchtenberger, Schrader & Weir, 1956; Leidl & Stolla, 1969; Gledhill, 1970), although no actual measurements of sperm head dimensions were recorded in some of these studies. Expected variations in the DNA content, as in Xand Y-bearing spermatozoa can be detected chemically but not morphologically (Beatty, 1970; Evans, 1972). In an attempt to clarify the relationship between genome and head size, Feulgen-DNA measurements were made in strains of mice in which variations in morphology have been reported (Beatty & Sharma, 1960). Four inbred strains were studied : A, JBT, CBA and C57. The spermatozoa of these strains are known to differ from each other in the size and shape of the head and midpiece. If head area alone is considered, A > CBA > JBT > C57, but the only significant differences are those involving C57 (see Table 1). Two males from each strain, aged between 9 and 11 weeks, were used. Spermatozoa from the ductus deferens were dispersed in a small volume of saline and smeared on glass slides. Each slide (A series) bore a total of eight * Present address: The Department of Zoology, Australian National University, Canberra, A.C.T., Australia. t Agricultural Research Council Unit of Animal Genetics.