Population & community ecology
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  Gene flow
  Topics relating to population & community ecology include gene flow, considered here, and INTERACTIONS WITH KELPS & SEA OTTERS, REMOVAL-TYPE STUDIES, and MASS MORTALITIES considered in other sections.
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Research study 1
 

map showing study sites for assessment of latitudinal growth differences in purple urchins Strongylocentrotus purpuratusInvertebrate species with long-lived planktotrophic larvae such as sea urchins are predicted to have widespread gene flow with homogeneous population characteristics.  However, differences are known to exist in demographic characteristics such as growth, reproduction, and life-span, and the question arises as to whether these have a genetic basis or are they phenotypic responses to environment.  This question is addressed at the San Diego State University marine laboratory in Mission Bay, California using data from 3 populations of purple urchins Strongylocentrotus purpuratus at Vancouver Island (British Columbia), San Diego (California), and Punta Baja (Mexico). The author marks specimens at each location using tetracycline to assess growth over a 1-yr period, but the more interesting part of the study is the collection of sea urchins from each site and their maintenance at Mission Bay at the same temperature and food regimen (kelp Macrocystis pyrifera is provided).  The urchins are left undisturbed for 10wk, then tagged with tetracycline marker.  At 4-5wk intervals individuals are graph showing dry masses of gonads of purple urchins Strongylocentrotus purpuratus at 3 locations on the west coastremoved for gonad and growth measurements. 

Results show that a similar pattern exists in the 3 populations with respect to gonadal growth, with spawning in February, increase in gonad mass through the spring and summer, and then a smaller spawning in September. Data for growth of test for the 3 populations also show a similar seasonal pattern.  The results for this part of the study show that whatever factors influence the reproductive and body-growth cycles have the same effect regardless of geographic location.  The author concludes that the data do not support an hypothesis of genetically based resource-allocation differences in S. purpuratus along its latitudinal distribution.  Russell 1987 J Exp Mar Biol Ecol 108: 199.

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Research study 2
 

Sea-urchin species have broad geographic ranges, high fecundity, and pelagic larvae that may be transported over large distances.  One west-coast species, Strongylocentrotus purpuratus, occupies a range extending from Baja California to southeastern Alaska, while another, S. droebachiensis, photograph of a purple sea-urchin Strongylocentrotus purpuratus eating a piece of seaweedoccurs in the northern coastal waters of both Pacific and Atlantic oceans.  To what extent will mitochondrial DNA vary both within and among the 2 species in view of these broad distributional ranges?  This is examined by researchers using specimens of S. purpuratus from San Francisco, California and San Juan Island, Washington, and specimens of S. droebachiensis from areas of Puget Sound, Washington, Walpole, Maine, and St. John’s, Newfoundland.  Some collections are repeated after a year to add a temporal element to the study.  Results for 38 S. purpuratus show no significant geographical structure within the 20 mtDNA genotypes and 4 clades found.  All 4 clades occur in both locatlities and in both years.  For S. droebachiensis, only 6 mtDNA genotypes are found among 41 individuals from all localities sampled, and 80% of individuals belong to 2 of these (with only 0.2% divergence between them).  The divergence though small, is significant, and suggests recent, but not continuous, migration.  The authors remark that the spread of larvae over large distances probably acts as a buffer in sea urchins to minimise genetic differences. Palumbi & Wilson 1990 Evolution 44: 403.


Purple urchin Strongylocentrotus purpuratus
eats a piece of seaweed 1X

 

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Research study 3

 

A later investigation using molecular methodologies provides a somewhat different picture of genetic make-up in west-coast populations of Strongylocentrotus purpuratus.  The study focusses on 10 sites (see yellow dots on map) to the north and south of Point Conception, California, where offshore movements of currents and current cycling could disrupt an homogeneous distribution of population characteristics.  The results of allozyme and mitrochondrial DNA differentiation in S. purpuratus at these sites show significant population subdivision among both adults (>20mm test diameter) and recruits (<20mm), and between adults and recruits. In fact, the data reveal a genetic mosaic, where differentiation over large distances is exceeded by differentiation over much shorter geographic distances.  While the authors expected to show a break in haplotype frequencies at Point Conception, they note that this is not supported by the data.  They do show a modest break at a point about 300km south of Point Conception, but are unable to pinpoint a cause.  Edmands et al. 1996 Mar Biol 126: 443.

NOTE  Point Conception marks the distributional limits of numerous species.  It is here that the cold southward-moving California current diverts offshore, allowing warmer water to accumulate along the southern coast

NOTE  the researchers sampled 2 sites at Laguna Beach, shown here as one

map of southern California showing collecting locations for purple urcchins Strongylocentrotus purpuratus for genetics study
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Research study 4
 

map showing sampling sites for a study on genetic differentiation in sea-urchins Strongylocentrotus franciscanus along the coast of CaliforniaThus, despite having long-lived planktonic larve (50-150d) and therefore decreased potential for significant genetic differentiation to occur among populations, at least one species of Strongylocentrotus does show such differentiation (see Research Study 3 above).  Whether this occurs with S. franciscanus along a similar latitudinal range of west-coast habitats is the subject of a study done at the Scripps Institution of Oceanography, La Jolla, California.  The researchers analyse allozyme data for several adult populations located along 1000km of California coastline, and investigate potential differences between “new” recruits of <2yr old and <30mm test diameter, subadults of 31-60mm, and adults of >60mm in a few discrete populations in northern California (see map).  Results show significant population heterogeneity in allelic frequencies among the adult populations.  No geographic pattern is evident, and neighbouring populations sited <100km from one another are often more genetically differentiated than distant populations.  Moreover, recruits differ significantly from adults at the same locale and also show significant between-year variation.  These surprising results indicate that the larval pool of S. franciscanus is not well mixed geographically, even on spatial scales of less than 20km, despite a planktonic existence of long-duration.  In showing that larval cohorts may not photograph of several juvenile sea-urchins Strongylocentrotus franciscanus sheltering under the spine canopies of adultstravel as far as once thought, the researchers provide valuable advice for those advocating establishment of harvest reserves to sustain long-term continued commercial harvest.  Specifically, they warn against creating one or just a few harvest reserves with the intent of supplying long stretches of downstream coastline.  Moberg & Burton 2000 Mar Biol 136: 773.


Juvenile sea-urchins Strongylocentrotus franciscanus are not always
commonly seen. Here, several shelter under the spine canopies of adults 0,5X

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Research study 5
 

map of study sites for sea-urchin genetics studyA research group from University of Caifornia, Santa Barbara, working at the same time as those in Research Study 4 but on a much larger scale (see map), comes to a similar conclusion regarding genetic subdivision in red urchins Strongylocentrotus franciscanus.  Individuals from 6 geographic locations between Alaska and Baja California are sequenced.  In total, 14 unique alleles are identified, with a maximum of 8 alleles being present at a given site.  Although there is some “moderate” sequence diversity, the authors are unable to reject a null hypothesis of random mating throughout the species range.  There are no significant differences in geneotype frequencies and no indication of a genetic break among the sites. Thus, at least for the bindin gene, there is high gene flow.  The authors remark that with a planktonic larval stage as long as 15-18wk, a pluteus larva in the California current could theoretically be transported for 1000km before settling, thus contributing to genetic homogeneity on a broad scale. However, based on their results and those of the La-Jolla research group, the authors suggest that 2 management plans for the sea-urchin fishery may be needed: one based on a west-coast scale; the other, on a regional site-specific scale.  Debenham et al. 2000 J Exper Mar Biol Ecol 253: 49.

NOTE  DNA sequence data from a 273-base-pair region of the bindin gene, encoding a sperm-fertilisation protein.  The authors are confident from other studies that the bindin locus is sufficiently polymorphic to detect genetic structure

NOTE  the single exception to this is the Alaska site, where non-random mating may have contributed to a degree of genetic variability

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Research study 6
 

mixed group of purple and red urchinsA study complementary to the ones above also addresses genetic variability in west-coast sea urchins, specifically, in purple urchins Strongylocentrotus purpuratus in California.  The premise being tested is that a marine species with a large individual reproductive output must realise only a tiny fraction of recruitment of its larval cohort each spawning season.  Thus, recruitment is like a sweepstakes, with chance events determining which adults are ultimately successful in passing on their genotypes.  Two predictions of such a sweepstakes reproductive-success hypothesis are that sea-urchin recruits would contain reduced genetic variation relative to the adult population and that cohorts of sea-urchin recruits would be genetically differentiated.  This is tested by analysing mitochondrial DNA sequences from 283 recently settled S. purpuratus (1-14d old) from 16 recruitment events spanning 7 years and 7 locations in California (32-39 N latitude).  Results, however, indicate that haplotype numbers and diversities show little evidence of reduced genetic variation in the recruits relative to that observed in a previous study of adults.  With respect to the second prediction, the authors note that different cohorts of recruits are in some cases mildly differentiated from each other.  The authors conclude that their data fail to provide convincing support for the sweepstakes reproductive-success hypothesis within the detection limits of their sampling.  Flowers et al. 2002 Evolution 56: 1445.

 

 

Mixed assemblage of purple,
green, and red sea urchins 0.5X

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Research study 7
 

cartoon of man viewing a purple urchinRecently, a large consortium of researchers has determined the complete genome of the purple sea-urchin Strongylocentrotus purpuratus.  The genome encodes 23,300 genes, comparable to the number in vertebrates, and these include many genes previously thought to associated exclusively with vertebrates.  The genome provides an evolutionary outgroup for the chordates and yields insights into the evolution of the Deuterostomia.  Additionally, the researchers find sea-urchin homologues for several sensory proteins related to hearing, chemosensation, and vision in vertebrates, which suggest hitherto unknown sensory capabilities in echinoids.  The genetic organisation of the sea-urchin immune system is extraordinarily complex and differs from any animal yet studied.  Many pathways are shared by sea urchins and humans, and the sea-urchin genome includes a large number of gene orthologues for human diseases.  Common patterns of genes in humans and sea urchins cover a surprising diversity of systems including nervous, endocrine, muscle, and skeletal.  Sea Urchin Genome Sequencing Consortium 2006 Science 314: 941.

NOTE  the sequencing project involved close to 50 research institutes and granting agencies, and hundreds of participating research scientists

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Research study 8
 

photographs of sperm heads from sea-urchins Strongylocentrotus droebachiensis from different parts of its distributionThe circum-arctic distribution of green sea-urchins Strongylocentrotus droebachiensis raises the question of genetic divergence relating to reproduction between Atlantic and Pacific populations.  The investigation, conducted at several locations in Europe (Bergen and Svalbard, Norway; Kristineberg, Sweden) and North America (Friday Harbor Laboratories, Washington and New Hamshire), primarily involves differences in sperm morphology.  This character is selected on the basis of known rapid evolution of traits in animals related to fertilisation.  Results show several significant differences, including sperm size, sperm-head shape, and content and location of filamentous actin.  Additionally, significant genetic divergence among the populations is evidenced by nuclear and mitochondrial DNA sequencing.  The authors suggest that such differences may relate to divergence in gamete compatibility within S. droebachiensis, and may represent the first stages of speciation.  Marks et al. 2008 Biol Bull 215: 115.

NOTE  sequencing of 16 alleles of the nuclear gene for sperm bindin, a protein that attaches activated sperm to the egg, plus sequencing of a section of the mitochondrial gene for ATPase subunit 6

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