Mytilus protamine-like sperm-specific protein genes are multicopy, dispersed, and closely associated with hypervariable RFLP regions.

Protamine-like sperm packaging proteins replace somatic histones during spermatogenesis, and although the proteins have been well-characterized in many marine invertebrate species, little is known of the arrangement of the genes. The research described here was designed to determine the sequence and structure of the protamine-like PL-III (or "phi 1") gene in marine mussels (Mytilus spp). The PL-III sequence was found to be extremely variable not only among the closely related Mytilus species, but also within species and populations. The variation observed among eight PL-III sequences from a single individual indicated that PL-III was probably multiple-copied. Southern analysis confirmed that PL-III, and another protamine-like gene, (PL-II), were multicopy and dispersed, as well as associated with a hypervariable element. Some PL-III genes are also arranged in nontandem clusters, and the spacer regions are probably the source of the hypervariable nature of the Southern blots. The arrangement of the protamine-like genes in Mytilus appears to be closer to that reported for histones than protamines; however, their association with a hypervariable element is novel.

Evidence for intragenic recombination within a novel genetic marker that distinguishes mussels in the Mytilus edulis species complex.

We have used the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) techniques to design two genetic markers for blue mussels in the Mytilus edulis species complex. Both of these markers target the gene encoding the mussel polyphenolic adhesive protein. The first marker, Glu-5', is highly differentiated among and can be used to identify the three blue mussel species, M. edulis, M. galloprovincialis and M. trossulus. The second marker, Glu-3', can identify M. edulis and M. galloprovincialis. Using these markers we have demonstrated that hybrid mussels from Whitsand Bay, UK carry alleles for this gene that are the products of intragenic recombination. The high frequency (10 per cent) of these recombinant alleles within the hybrid population suggests that recombination is fairly frequent within this gene or that hybridization between M. edulis and M. galloprovincialis is substantial and has been occurring over considerable evolutionary time. The two novel genetic markers, Glu-5' and Glu-3' will be invaluable in additional studies regarding the importance of hybridization among blue mussels.

The effects of natural hybridization on the regulation of doubly uniparental mtDNA inheritance in blue mussels (Mytilus spp.).

Blue mussels in the Mytilus edulis species complex have a doubly uniparental mode of mtDNA inheritance with separate maternal and paternal mtDNA lineages. Female mussels inherit their mtDNA solely from their mother, while males inherit mtDNA from both parents. In the male gonad the paternal mtDNA is preferentially replicated so that only paternal mtDNA is transmitted from fathers to sons. Hybridization is common among differentiated blue mussel taxa; whenever it involves M. trossulus, doubly uniparental mtDNA inheritance is disrupted. We have found high frequencies of males without and females with paternal mtDNA among hybrid mussels produced by interspecific matings between M. galloprovincialis and M. trossulus. In contrast, hybridization between M. galloprovincialis and M. edulis does not affect doubly uniparental inheritance, indicating a difference in the divergence of the mechanisms regulating mtDNA inheritance among the three blue mussel taxa. Our data indicate a high frequency of disrupted mtDNA transmission in F1 hybrids and suggest that two separate mechanisms, one regulating the transmission of paternal mtDNA to males and another inhibiting the establishment of paternal mtDNA in females, act to regulate doubly uniparental inheritance. We propose a model for the regulation of doubly uniparental inheritance that is consistent with these observations.

Ecological interaction between sympatric Mytilus species on the west coast of Canada investigated using PCR markers.

M. californianus is the dominant marine mussel species on exposed rocky shores, while M. trossulus is usually the dominant mussel species in more sheltered waters on the west coast of North America. Since these species are physical indistinguishable when small (< 10.0 mm), we developed two polymerase chain reaction (PCR) -based markers to discriminate between them. Using these markers, we identified mussels taken from an exposed coast (n = 114), a sheltered harbour (n = 80), and an upper-intertidal pool (n = 42) on the west coast of Vancouver Island, British Columbia, Canada. M. californianus were found only on the open coast. Small M. trossulus (< 20.0 mm) were common to all three sample sites, but were extremely rare at larger sizes (> 20.0 mm) on the open coast. Our results indicate that M. californianus are excluded from sheltered waters via early life factors, while M. trossulus are excluded from the open coast due to mortality later in life.

Evolutionary relationships among the male and female mitochondrial DNA lineages in the Mytilus edulis species complex.

A novel form of mitochondrial DNA (mtDNA) inheritance has previously been documented for the blue mussel (Mytilus edulis). Female mussels inherit their mtDNA solely from their mother while males inherit mtDNA from both their mother and their father. In males, the paternal mtDNA is preferentially amplified so that the male gonad is highly enriched for the paternal mtDNA that is then transmitted from fathers to sons. We demonstrate that this mode of mtDNA inheritance also operates in the closely related species M. galloprovincialis and M. trossulus. The evolutionary relationship between the male and female mtDNA lineages is estimated by phylogenetic analysis of 455 nucleotides from the large subunit ribosomal RNA gene. We have found that the male and female lineages are highly divergent; the divergence of these lineages began prior to the speciation of the three species of blue mussels. Further, the separation between the male and female lineages is estimated to have occurred between 5.3 and 5.7 MYA.

Genetic control of juvenile growth rate in mice: variation between a congenic strain and its background strain.

Studies on the genetic regulation of growth are confounded by the multigenic basis of growth. There is a need to isolate simplified genetic systems for the study of growth. We compared two closely related mouse strains and their F1 hybrids with regard to birth and weaning weights. The strains we used were C57BL/6 (B6) and a congenic derivative of B6 (HW54) that contains a short segment of BALB/c chromosome 7 spanning the H-24 and Gpi-1 loci. Despite the genetic similarity of these strains, they differed significantly in both birth and weaning weights. At birth, B6 pups were on average as much as 6.6% heavier than were pups from HW54. By the time of weaning, this trend was reversed; HW54 pups were as much as 13.8% heavier than were B6 pups. (B6 x HW54)F1 hybrids were intermediate between the parental strains in birth weight but were identical to B6 animals at weaning. An analysis of the F2 generation suggested that postnatal growth differences between B6 and HW54 are probably dependent on the maternal genotype. These strain-specific growth rates result from polymorphism at a restricted portion of the genome and represent a highly simplified system for the study of the genetics of growth.

Dominance in physiological phenotypes and fitness at an enzyme locus.

Aminopeptidase-I allozymes, which are products of the Lap locus in the marine mussel, Mytilus edulis, differ in their catalytic efficiencies. These biochemical differences result in genotype-specific rates of change in the free amino acid pool, that is, cell volume regulation, when mussels are subjected to changes in salinity. A high degree of dominance was found among genotypes for these biochemical and physiological phenotypes. Selection models that incorporate dominance adequately predict observed genotypic properties at the Lap locus among natural populations that exhibit clinical allele frequency. This suggests that a high degree of dominance for fitness must also occur at this locus in natural populations. These results provide additional evidence that the maintenance of an allele frequency cline is operating by natural selection at the Lap locus.