What do we know about the genetics of anguillid eels?

1. What do we know about the genetics of anguillid eels? Eric Hallerman Virginia Polytechnic Institute and State University

2. H0: Given that all American eels spawn in the Sargasso Sea, we should expect panmixia. Is this the case?

3. Williams et al. (1973) examined elvers, juveniles, adults from FL, NC, NY, NS, NFLD • Screened polymorphic enzyme markers at 5 loci • Found geographic variation in allele frequencies within life stages • Showed allele frequency differences ~10% among samples over a 20o latitudinal range • What would we see over the 50o latitudinal range of the species?? • There are no self-sustaining localized populations – hence, this differentiation must arise anew every generation! • Clinal SDH and ADH variation in elvers, PHI in adults – hence, different selective pressures on different life stages Williams, G.C., R.K. Koehn, and J.B. Mitton. 1973. Genetic differentiation without isolation in the American eel, Anguilla rostrata. Evolution 27:192-204.

4. Koehn and Williams (1978): Are the selective regimes temporally stable? • Elvers and juveniles PR, FL, NY, NS, NFLD various years • Screened for variation at SDH, PHI, ADH loci • Spatial heterogeneity was stable at SDH and PHI loci • Clinal SDH variation was identical in recruits and residents – hence, selection operated before recruits entered rivers • Clinal PHI variation seen only in residents – hence, selection operated after recruits took up residence in rivers • Selective pressures at SDH and PHI loci are temporally stable • ADH variation homogeneous across North America, but varied among years • Selective regimes are dynamic Koehn, R.K., and G.C. Williams. 1978. Genetic differentiation without isolation in the American eel, Anguilla rostrata. II. Temporal stability of geographic patterns. Evolution 32:624-637.

5. Williams and Koehn (1984) reviewed the meristics and genetics of A. rostrata and A. anguilla. Key aspects: There is no evidence of self-sustaining local populations within putative species The only genetic evidence for distinct species of Anguilla is from variation at the MDH-2 locus Intercontinental genetic differentiation is comparable to that observed among conspecific populations, not distinct species There is intergradation and gene flow at least from American to European forms: • Species diagnostic character - number of vertebrae • Low vertebral counts sometimes seen in Icelandic and Swedish localities • Icelandic eels have intermediate vertebral counts and MDH alleles from both types W&K concluded that American and European eels are partially separated breeding populations of one species, A. anguilla Skewed sex ratios are often observed: • Sex determination may have both genetic and environmental components • Suggested environmental manipulation and common garden experiments Williams, G.C., and R.K. Koehn. 1984. Population genetics of north Atlantic catadromous eels (Anguilla). Pages 529-560 in B.J. Turner, ed. Evolutionary genetics of fishes. Plenum Press, NY. Williams, G.C. et al. 1984. Icelandic eels: evidence for a single species of Anguilla in the North Atlantic. Copeia.

6. Avise et al. (1986): Would mtDNA show geographic variation in North Atlantic eels? • 9 sites: LA, FL, GA-2, NC, RI, ME, Ireland, England • Restriction site variation for 14 informative enzymes • A. rostrata homogeneous – one haplotype predominated in all North American samples; variants differed at 1 or 2 restriction sites; mean divergence p ~0.001 • A. anguilla more variable; greater divergence among haplotypes; mean divergence p ~0.008; no differentiation between Irish and English populations • Comparison of species: 11 of 14 enzymes showed distinct digestion profiles. Hence, species very distinct, p ~0.037. • Hypotheses: • A. rostrata and A. anguilla spawn sympatrically, but do not interbreed. • A. rostrata and A. anguilla spawn allopatrically, such that larval dispersal leads to observed continental distribution Avise, J.C., et al. 1986. Mitochondrial DNA differentiation in north Atlantic eels: population genetic consequences of an unusual life history pattern. Proc. Natl. Acad. Sci. USA 83:4350-4354.

7. Avise et al. (1988) assumed large population size and high gene flow within American eel • Estimated time since the most recent common ancestor on basis of data from Avise et al. (1986) • Molecular distances among mtDNA lineages were small, 2-3 orders of magnitude less than predicted on basis of current population size • Hypothesis 1: MtDNA evolution is slowed in this species • Hypothesis 2: Long term Nef = 5,500 is much less than current Ne • With high fecundity (> 1 million eggs), there is great opportunity for high variance of progeny numbers among families, decreasing Ne, reducing variability • Ne << N Avise, J.C., et al. 1988. Current versus historical population size in vertebrate species wit high gene flow: A comparison based on mitochondrial DNA lineages and inbreeding theory for neutral mutations. Mol. Biol. Evol. 5:331-344

8. American and European eels differ only in number of vertebrae and geographic distribution • Are A. rostrata and A. anguilla really distinct species? • What about eels with low vertebral numbers in Iceland and, rarely, Europe? • Avise et al. (1990) examined eels from four sites in Iceland for mtDNA, MDH-2, and vertebral counts • Icelandic eels include interspecific hybrids at sites 1 and 2; hybridization in both directions with regard to sex • Genes are flowing from A. rostrata into A. anguilla background. 2-4% of gene pool is of American eel ancestry • Migration behavior and morphology of hybrids intermediate to parents, suggesting additive genetic mode of expression • Authors hypothesize that all Atlantic eels one derive from a single population that became separated into two forms ~1.5 mya; divergent gene pools arose and maintained by disruptive selection for larval development rate and by spawning in separate areas of the Sargasso Sea Avise, J.C, et al. 1990. The evolutionary genetic status of Icelandic eels. Evolution 44:1254-1262.

9. Can we learn anything pertinent from studies of other anguillid eels? • Japanese eel, A. japonica • Neither isozymes nor mitochondrial DNA show geographic genetic differentiation • Taniguchi, N., and K. Numachi. 1978. Genetic variation of 6PGDH, IDH, and GOT in the liver of Japanese eel. Bull. Jpn. Soc. Sci. Fish.44:1351-1355. • Song et al. 1994. Population structure of the Japanese eels, Anguilla japonica. Molecular Biology and Evolution 11:250-260.

10. Can we learn anything pertinent from studies of other anguillid eels? • European eel, A.anguilla • Allozymes – no evidence against hypothesis of panmixia: • DeLigny, W., and E.M. Pantelouris. 1973. Origin of the European eel. Nature 246:518-519. • Comparini, A.., et al. 1977. Genetic control and variability of phosphoglucose isomerase (PGI) in eels of the Atlantic Ocean and the Mediterranean Sea. Mar. Biol. 43:109-116. • Rodino, E., and A. Comparini. 1978. Genetic variability in the European eel, Anguilla anguilla L. in Battaglia and Beardmore, eds. Marine organisms: Genetics, ecology, and evolution. Plenum Press, NY. • But variation is clinal: • Pantelouris, E.M., et al. 1970. Genetic variation in the eel. Genet. Res. Camb. 16277-284. • MtDNA – no evidence against hypothesis of panmixia: • Lintas C., et al. 1998. Genetic variation in the European eel (Anguilla anguilla). Mol. Mar. Biol. Biotech. 7:263-269.

11. Can we learn anything pertinent from studies of other anguillid eels? • Microsatellite DNA • Wirth and Bernatchez (2001) examined variation at 7 microsatellite loci among 13 populations from north Atlantic, Baltic Sea, Mediterranean Sea • 13-21 alleles per locus • There was weak but significant genetic differentiation (FST = 0.0017), especially of Mediterranean vs. Baltic and North Sea populations • Significant correlation with distance • Authors suggest stocks are maintained by spawners arriving at different times at spawning areas and larval homing (tenuous?) • Authors did not consider effect of selection occurring after larval dispersal as in A. rostrata • Note that differentiation was observed only with powerful microsatellite markers Wirth, T., and L. Bernatchez. 2001. Genetic evidence against panmixia in the European eel. Nature 409:1037-1040.

12. Research questions relevant to management of A. rostrata:Genetic issues • What is genetic variation across the entire range of A. rostrata from Surinam to Greenland? • Would screening of microsatellite DNA markers reveal new findings? • The A. anguilla microsatellite primers would probably work for A. rostrata

13. What if we find geographic genetic variation? • Case study: Salmonids • Many offshore fisheries are actually mixtures of distinct spawning populations • Ex: Chinook salmon troll fishery Miller, M., et al. 1983. Analysis of chinook stock composition in the May 1982 troll fishery off the Washington coast: an application of the genetic stock identification method. Washington States Dept. of Fisheries Technical Rept. No 74. 27 pp.

14. What if we find geographic genetic variation? • The most likely composition of a mixed stock fishery can be estimated: Milner, G.B., et al. 1985. A genetic method of stock identification in mixed populations of Pacific salmon, Oncorhynchus sp. Mar. Fish. Rev. 47:1-8.

15. What if we find geographic genetic variation? Harvest is managed so as to protect the demographically most vulnerable population(s) Miller, M., et al. 1983. Analysis of chinook stock composition in the May 1982 troll fishery off the Washington coast: an application of the genetic stock identification method. Washington States Dept. of Fisheries Technical Rept. No 74. 27 pp.

16. Research questions relevant to management of A. rostrata:Ecological -demographic issues • Are population trends similar for both American and European eels (perhaps with a 1-2 year lag)? How would the answer affect inference of causal factors? • Could the apparent decline of North American eels be due in part to harvest of elvers for aquaculture?