Unequal partitioning of bovine mitochondrial genotypes among siblings.

Two polymorphic mitochondrial DNA genomes, differing by a single Hpa II restriction site, are present at significantly different levels in tissue of three sibling dairy cows. The relative ratio of the two heteroplasmic molecules varies 3-fold among these three animals and documents a rapid segregation of mitochondrial genotypes in mammals. DNA sequencing shows the difference is due to a single guanine at position 364 in bovine mitochondrial DNA. A model involving unequal partitioning of the two amplified mitochondrial DNA species during the early cell divisions of the embryo can explain the appearance of such variation in heteroplasmic sibling animals. The model provides a basis for understanding the rapid DNA sequence variation observed in vertebrate mitochondrial DNA despite its high copy number and strict maternal inheritance.

Partial protection of lambs against Haemonchus contortus by vaccination with a fractionated preparation of the parasite.

Florida Native lambs, less than 6 months of age, were successfully vaccinated against Haemonchus contortus with a high mol. wt fraction (greater than 30,000 daltons) derived from a somatic extract of H. contortus larvae (SEL) and excretions and secretions (ES) of larvae isolated during in vitro development from the infective 3rd to 4th stage. A 59% reduction in adult worm numbers was obtained in vaccinates compared to naive lambs following challenge. The protection in vaccinated lambs was similar to that seen in lambs exposed to a primary infection of H. contortus larvae which had been cleared with anthelmintic prior to the challenge infection. The unfractionated SEL/ES preparation and a low mol. wt fraction gave no significant protection against challenge infection.

Heterogeneous mitochondrial DNA D-loop sequences in bovine tissue.

Mitochondrial DNA from bovine tissue contains heterogeneous sequences located within an evolutionary conserved cytosine homopolymer sequence near the 5' end of the D-loop region. This part of the mammalian mitochondrial genome is known to contain the origin of heavy strand DNA synthesis and the major transcriptional promoter for each strand. Nucleotide sequence analysis of cloned DNA and electrophoretic analysis of appropriate small fragments from animal tissue reveal a population of length polymorphs containing from nine to 19 cytosine residues. No individual length species represents more than 40% of the population. These data imply a state of significant intraanimal mtDNA sequence heterogeneity, which most likely occurs intracellularly as well. The localization of variability to a homopolymer run suggests that replication slip-page generated the sequence population. We also report that when recombinant clones containing this region are repeatedly passaged in E. coli, they begin to regenerate length variation similar to that seen in animal mtDNA.

Nucleotide sequence evidence for rapid genotypic shifts in the bovine mitochondrial DNA D-loop.

Mitochondrial DNA (mtDNA) is unusual in its rapid rate of evolution and high level of intraspecies sequence variation. The latter is thought to be related to the strict maternal inheritance of mtDNA, which effectively isolates within a species mitochondrial gene pools that accumulate mutations and vary independently. A fundamental and as yet unexplained aspect of this process is how, in the face of somatic and germ-line mtDNA ploidy of 10(3) to 10(5) (refs 4, 5), individual variant mtDNA molecules resulting from mutational events can come to dominate the large intracellular mtDNA population so rapidly. To help answer this question, we have determined here the nucleotide sequence of all or part of the D-loop region in 14 maternally related Holstein cows. Four different D-loop sequences can be distinguished in the mtDNA of these animals. One explanation is that multiple mitochondrial genotypes existed in the maternal germ line and that expansion or segregation of one of these genotypes during oogenesis or early development led to the rapid genotypic shifts observed.