Genetic, molecular, and cytological characterization of the paternal effect gene, paternal loss, of Drosophila melanogaster
Mutations in the paternal effect gene, paternal loss (pal) of Drosophila melanogaster cause loss of only the paternally inherited chromosomes during early embryogenesis. In an effort to better understand the role of the paternal contribution to embryogenesis, the genetic and molecular properties of mutations in the pal gene were characterized. Comparisons of the original $pal\sp1$ allele, a newly isolated dysgenic allele, $pal\sp2$, and deficiencies of the locus defined pal as a strict paternal effect gene. The weak penetrance of null mutations in the pal gene suggested that the pal function is partially redundant with other gene functions. The pal gene was cloned showing that the gene encodes a small mRNA (0.9 kb) expressed only in the male germline. Expression of the pal mRNA is localized in early primary spermatocytes in a polar perinuclear cap and is homogeneous in later spermatocytes. Sequence analysis of the pal cDNAs and genomic clones predicted that the encoded protein is a small highly basic protein. Immunolocalization studies showed that the protein is present in condensing, elongating spermatids and in mature sperm. Preliminary results suggested that the pal protein may enter the embryo in the sperm nucleus. I propose a model in which the pal protein is a sperm-specific chromosomal protein required for the normal condensation of the sperm nuclei or for the decondensation of the sperm nuclei in the embryo. Loss of the pal protein may disrupt the exchange of sperm-specific chromosomal proteins for maternally provided proteins in the first few minutes of embryogenesis. Defects in this transition may result in late or improperly decondensed paternal chromosomes and their subsequent misbehavior. Progeny of pal fathers showed early embryonic defects in mitosis, including chromosomes off the metaphase plate, chromosomes lagging at anaphase and distorted spindles, as well as the presence of micronuclei prior to the first mitosis. These defects may arise in the absence of the pal protein due to an altered interaction of paternal chromosomes with cellular components such as the cytoskeleton or nuclear envelope.
- Genetics