title for learn-about section of A SNAIL'S ODYSSEY
  Population & community ecology
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  Extent of genetic differentiation
  This part of population & community ecology deals with extent of genetic differentiation, while topics of MUSSEL-BED DIVERSITY, COMMUNITY SUCCESSION, INTRASPECIFIC COMPETITION, and INTERSPECIFIC COMPETITION are considered in other sections.
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Research study 1
 

map showing collecting sites for study of genetic differentiation in mussels Mytilus californianus and M. trossulus on the west coast of North Americagraph showing variation in frequency of the LAP allele in mussels Mytilus californianus and M. trossulus from various locations along the west coast of North AmericaA comparison of genetic differentiation in mussels Mytilus trossulus1 and M. californianus collected at 9 sites2 from Santa Barbara, California to Torch Bay, Alaska (see map on Left) shows greater among-site difference and greater levels of polymorphism in the broader habitat-occupying (eurytopic) M. trossulus than the in the more narrow habitat-occupying (stenotopic) M. californianus at 2 enzyme loci3 (see graph on Right for LAP allele).  A comparison of genetic differentiation in M. californianus on a scale of meters (on a single rock), on a scale of kilometers (on San Juan Island, Washington), on a scale of 100km (Strait of Juan de Fuca), and on a scale of 1000km (along the entire west coast), is minimal and can be explained by microhabitat differences in a single rocky area.  The authors explain the lower genetic differentiation of west-coast Mytilus spp., in comparison with the strong geographic differentiation in the related M. edulis on the east coast of North America, to the steeper latitudinal thermal gradient present on the east coast. Levinton & Suchanek 1978 Mar Biol 49: 363.

 

NOTE1  the authors refer to this species as M. edulis, which is now known not to occur on the west coast of North America.  The authors have not sampled mussels from south of Santa Barbara, so M. galloprovincialis is likely not in the mix

NOTE2  the samples are taken as close as possible to the midpoint of the vertical range of each mussel bed

NOTE3  electrophoretic separation of the enzymes L-leucyl-peptide-hydrolase-1 and LAP-2 in preparations of the digestive gland

 
Research study 2
 

schematic showing degree of genetic similarity in mussels Mytilus galloprovincialis from clean and "impacted" sites in CaliforniaMost ecological assessments of the impact of pollution on shore invertebrates involve measures of changes in biomass and diversity, and other less visible effects such as loss of genetic diversity within a population have gone unnoticed.  To assess whether such an effect can be measured in existing populations, researchers in southern California compare genetic diversity1 among 7 populations of mussels2 Mytilus galloprovincialis.  Three populations inhabit relatively “clean” sites3 and 4 populations inhabit “impacted” sites exposed to heavy industrial, boating activity, or other pollution sources.  Although all populations are genetically similar, enough differences exist to suggest that populations from "impacted4" sites have lower genetic diversity than populations from clean sites.  Specifically, the impacted sites are more genetically similar to one another than they are to the clean sites, while clean sites are much more different from one another. Ma et al. 2000 Mar Envir Res 50: 559.

NOTE1  the researchers use randomly amplified polymorphic DNA-polymerase chain-reaction methodology

NOTE2  barnacles Balanus glandula are also included in the study, but only mussels are considered here. The results for both taxa are similar

NOTE3  "impacted" sites are: PH Port Hueneme, FS Fire Station # 49 inner Los Angeles Harbor, FB Fire Staion #20 inner Long Beach Harbor, BI Balboa Island Newport Bay; "clean" sites are CA Catalina Island, NP near mouth of Newport Bay, DP outer harbor at Dana Point

NOTE4  known pollutants from these sites include heavy metals, pesticides, polycyclic aromatic hydrocarbons, and tributyltin

 
Research study 3
 

photograph of normal diploid complement of 28 chromosomes in musses Mytilus trossulusA group of Spanish researchers working out of the University of Victoria, British Columbia have reported incidences of natural polyploidy in bay mussels Mytilus trossulus from Esquimalt Lagoon, Vancouver Island.  Individuals of this species normally have a diploid chromosome count of 28, but 4 of 168 analysed in the study displayed a modal value of 76 chromosomes, with the highest number being 92.  The authors speculate on the cause of the polyploidy condition, and suggest that it may be a consequence of a haemocytic neoplasia condition known as “Summer Mortality Syndrome”.  González-Tizón et al. 2000 Genome 43: 409.

 

Normal complement of 28 chomosomes in
a diploid individual of Mytilus trossulus

 
Research study 4
 

map showing study sites in California for investigation of habitat preferences of mussels Mytilus trossulus and M. galloprovincialisgraphs showing temperature and salinity preferences for mussels Mytilus galloprovincialisIn the areas of overlap of native Mytilus trossulus with invasive M. galloprovincialis and their hybrids in central California, are there certain conditions of temperature and salinity that are more favourable to one or other with respect to survival?  This is investigated at 4 sites in San Francisco Bay and 4 in Monterey Bay that together provide a gradient from oceanic to estuarine conditions (see map on Left). 

Results over a 4-yr sampling period show that the proportion of M. galloprovincialis is greater where salinities are higher and maximum temperatures are lower, while the opposite is true for M. trossulus. The relationship to temperature is surprising, as the former species with its origins in the Mediterranean is usually thought to favour warm water, while the latter species is thought to favour cooler water.  Proportion of hybrids correlates neither with temperature or salinity.  These and other longer-term assessments reveal that while the hybrid zone lies generally between Monterey and Cape Mendocino (see map), within the 2 major bay systems the populations of the 2 parent mussel species and their hybrids are in constant flux.  Braby & Somero 2006 Mar Biol 148: 1249. 

 
Research study 5
 

map showing distributions of pure, hybrid, and back-cross mussels Mytilus spp. at 4 locations from southern British Columbia to San Diego, Californiamussel shell showing 4 measurements used in symmetry analysisIn a follow-up study1 to one on hybrids of bay-mussel Mytilus spp. around Vancouver Island, researchers assess the fitness of M. trossulus/M. galloprovincialis hybrids2 at several west-coast locations from southern British Columbia to southern California (see map).  Fitness of hybrids is defined here by the degree of “developmental stability”, that is, the extent to which shell growth on each side of the body is symmetrical. Deviation in symmetrical shell growth, also called “fluctuating3 asymmetry, is equated here with decreased competitive fitness, defined as the extent to which exogenous factors affect normal symmetrical growth. The accompanying diagram shows histogram comparing shell-development asymmetries in mussels Mytilus trossulus, M. galloprovincialis, and their hybrids and back-crosses at 4 sites along the west coast of North Americathe linear measurements taken to assess symmetry (see illustration upper Right; a 5th trait, not shown, is dry mass of each shell valve).  After statistical conversion, the differences between right and left shell traits are summed to give a proxy estimate of total shell asymmetry (see histogram lower Right).

The extreme outer bars in the histogram represent M. trossulus at Newport, Oregon and M. galloprovincialis at San Diego, California, while the inner bars represent asymmetries of these with hybrids found on Vancouver Island (Chemainus) and San Francisco. Results show significant “heterozygote deficiency”, that is, significant shell-development asymmetry/decreased fitness in some of the M. galloprovincialis hybrids from Chemainus in comparison with pure genotypes of M. trossulus and M. galloprovincialis, but an opposite tendency may be present for these hybrids in comparison with San Francisco M. trossulus.  The study is unique and interesting in that it provides a means for objective assessment of selective pressures that create and maintain these Mytilus species.  The authors note that the presence of at least 2 hybrid crosses at Chemainus that are less “developmentally stable” than the parental genotypes, is suggestive of at least some contribution to reproductive isolation among west-coast Mytilus species.  Springer & Heath 2007 Mar Biol Res 3: 182.

NOTE1 see section on IDENTIFICATION OF MUSSELS: MOLECULAR-GENETICS METHODS

NOTE2  hybrid frequency at each location is assessed using PCR-based nuclear-species markers. Three nuclear loci are used to assign species status: for details see the earlier paper. The hybrid categories shown here include both types of back-cross hybrids

NOTE3  when no significant right-left directionality is evident in the 5 shell traits examined, the condition is described as “fluctuating”

 
Research study 6
 

map showing study sites used in an investigation of fitness in mussels Mytilus trossulus, M. galloprovincialis, and their hybrids in British Columbiahistograms comparing fitness of mussels Mytilus trossulus, M. galloprovincialis, and their hybrids at 2 locations in British ColumbiaIn a somewhat more direct assessment of fitness amongst native, introduced, and hybrid bay mussels1, researchers in British Columbia place caged representatives of each group in local and remote2 sites in British Columbia, and monitor their growth and survival (as proxies for fitness) over a 3-mo period (see map).

Results are highly variable, but there is a trend of higher fitness3 of F1 hybrids at both sites (see histogram). Overall fitness is also significantly higher at the Quadra Island site, an area characterised by cleaner, colder, and more nutrient-rich water.  The demonstration here of a degree of “hybrid vigour” in these species is partially contradictory to the conclusions generated in a previous Research Study above by the same research group.  Shields et al. 2008 Mar Biol 154: 919; for more on the genetic structure of these mussel species in and around Vancouver Island see Shields et al. 2010 Mar Ecol Progr Ser 399: 211. 


NOTE
1  the species used are Mytilus trossulus (native), and M. galloprovincialis (introduced), and their F1 and back-cross hybrids

NOTE2  the collection site (local site) is at Ladysmith and the remote site is at Quadra Island (see map)

NOTE3  calculated as the arithmetical product of mean growth, measured as volume, times mean survival for each genotype. The data presented here represent just a small portion of the total data set

 
Research study 7
 

A later and more comprehensive study of genetic differentiation in sea mussels Mytilus californianus compares patterns of variation using 4 independent map showing collecting sites for mussels Mytilus californianus in a study of genetic differentiationtypes of genetic markers from individuals collected over 4000km of its distributional range (see map).  The authors could find no significant differences among localities or geographic distance from any of the markers, confirming that the species is genetically homogenous throughout its range.  The authors conclude that the photograph of zonation patterns of invertebrates on the west coast of Vancouver Island, British Columbiagenetic uniformity exhibited by M. californianus likely results from extensive gene flow and its open-coast habitat uniformity throughout its range.  Addison et al. 2008 Mol Ecol 17: 4222.

NOTE  allozymes, single-copy nuclear DNA markers, and DNA sequences from mitochondrial genomes of both sexes


Wave-exposed open-coast are
favoured by sea mussels Mytilus californianus. The mussels occupy
the irregular blue-black band in
the upper right portion of photo

 
Research study 8
 

map showing collection sites for mussel studygraph showing effect of acute heat stress on mussels Mytilus californianus survival over timeSea mussels Mytilus californianus inhabit rocky shorelines from Alaska to Baja California, a difference of some 4000km, and and experience large variation in temperature and other environmental conditions.  This raises the question of degree of local adaptation or phenotypic plasticity to thermal conditions over their range of latitudinal distribution.  The issue is investigated by researchers at Hopkins Marine Station, California by testing survival rates, thermal maxima of cardiac function, and metabolic capacities of individuals collected from mid-intertidal locations at 7 sites across 33o  of latitude (see map).  Results show that the most northern population, at Tatoosh Island, Washington has significantly lower survival rates than all other populations after acute heat stress (hourly increases of 8oC, followed by1h at 36oC; see graph), and also lowest thermal maximum of heart function, both features indicative of adaptation to cooler condtions.  Metabolic capacity, however, generally does not correlate with latitude.  Overall, the authors conclude that phenotypic plasticity does not explain differences in thermal tolerance, and that other factors, such as genetic differences may be involved.  Logan et al. 2012 Mar Ecol Progr Ser 450: 93.

NOTE  measured as heart-rate failure during acute heat stress and activity of an indicator enzyme, malate dehydrogenase

NOTE  mussels to be tested are held at common acclimation temperatures (12-13oC) for 6wk-6mo prior to experimentation.  A strong correlation of survival with latitude (p < 0.001) is found, and this trend remains even if the Tatoosh Island population is removed from the analysis

 
Research study 9
 

photograph of bed of date mussels Musculista senhousia courtesy Mel Lin, Singaporemap showing sampling sites for study on genetic connectivity in populations of date mussels Musculista senhousiaThe invasive date-mussel Musculista senhousia was introduced into the Puget Sound region in the 1920s in shipments of Japanese oysters Crassostrea gigas, and since that time has spread at least as far south as San Diego.  The mussels form aggregations often  numbering in excess of 100,000 individuals per square meter, and other native soft-sediment-inhabiting species may be negatively affected. The manner of its spread is investigated by researchers from California State University, Los Angeles who estimate the extent of genetic connectivity of individuals in 6 west-coast estuarine populations with individuals in 3 putative Asian source populations.  Results show that a warm-water genetic lineage from Tokyo/southern South Korea predominates in southern California, while a cold-water lineage from northeastern South Korea is more genetically similar to Puget Sound and more northern California populations.  The authors report a high genetic interchange among individuals in the southern California sites, and among individuals in the northern sites, but there is no gene flow (no shared haplotypes) between the northern and southern U.S. regions.  Comparison of the genetics data with several different iterations of water temperatures at the various sites suggests to the authors that incompatible water temperatures may be acting to limit post-introduction spread of M. senhousia.  Asif & Krug 2012 Biol Invasions 14: 1431. Photograph courtesy Mel Lin, Singapore.

NOTE  distribution of mitochondrial DNA and alleles of a nuclear intron.  Results for 2 other invaded sites, New Zealand and Mediterranean, are also included in the study, but not reported here. The researchers sample several other sites where mussels are absent, but these are not shown on the map

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