Neither a germ line-specific nor several somatically expressed genes are lost or rearranged during embryonic chromatin diminution in the nematode Ascaris lumbricoides var. suum

Abstract

Ascaris lumbricoides var. suum is a parasitic nematode of pigs. Its embryos undergo chromatin diminution between the third and fifth cleavages, resulting in the loss of about 30% of the DNA from all somatic precursor cells while the germ line DNA stays intact. Most of the eliminated DNA has been shown to be satellite sequences. Theodor Boveri [(1910) In “Festschrift fur R. Hertwig, III,” Vol. 3, pp. 131–214, Fischer] proposed that functions essential only to the germ line might be lost from the soma. We have examined this proposal by cloning a gene encoding the major sperm protein (MSP) using a cloned MSP gene from Caenorhabditis elegans as a probe. The MSP appears to be expressed only in the testis of Ascaris, as it is in Caenorhabiditis. Actin and αtubulin were also cloned to serve as somatically expressed gene controls. By probing Southern blots of somatic and germ line DNA with these cloned genes, it was found that none of them was lost or rearranged during chromatin diminution. Thus at least one germ line-specific gene is neither lost nor rearranged during chromatin diminution. We also found that the two nematode species differ widely in their numbers of both MSP and actin genes. Caenorhabditis has >30 MSP genes, but Ascaris has no more than three; whereas Ascaris has many more actin genes than Caenorhabditis.