Population genetics of Pseudobarbus phlegethon, Barbus calidus and Barbus erubescens (Teleostei: Cyprinidae) of the Olifants River System, Western Cape Province

[摘要] ENGLISH ABSTRACT:Endemic redfin minnows of the Olifants River System are under threat of extinction, mainlybecause of alien predatory fish, water extraction and pollution. The taxonomic position andbiogeography of redfins in relation to other barbs are uncertain. Enzyme electrophoresis wasemployed to assess genetic differentiation within and among certain Barbus andPseudobarbus taxa. Fourteen enzymes were routinely recorded from muscle tissue extracts,yielding a differential number of active loci. The latter is due to different ploidy levels.Polyploid species showed fewer loci than the tetraploid or hexaploid number of loci expectedjust after the polyploidy event. Barbus aenus (hexaploid) expressed 6.2 % of the duplicatedloci expected after the hexaploidy event. The tetraploid Pseudobarbus and serrated Barbusspp., expressed 37.5 % and 31.2 % of the duplicated loci expected after the tetraploidy event,respectively. This suggests that the ploidy event in Barbus aenus is more ancient than in theabove-mentioned tetraploid barbs. The similar amount of diploidization in Pseudobarbus andtetraploid serrated Barbus spp. investigated, suggests a shared ploidy event between these twolineages. The number of active loci is apparently not a good indication of the ploidy level ofAfrican barbs, as hexaploid Barbus aenus expressed less active loci than the tetraploid barbsinvestigated. Initial screening of allozyme loci yielded fixed allele differences at 22 loci, butpolymorphism only at seven. This suggests more potential for delineating species boundariesthan for assessing gene flow and genetic diversity of populations. Genetic differentiationwithin and among populations of Pseudobarbus phlegethon, Barbus calidus and Barbuserubescens were investigated by screening 27 allozyme loci. A clear divergence betweenPseudobarbus phlegethon populations from the Olifants and Doring Rivers was found (Nei'sunbiased genetic distance = 0.355; F-statistic for subpopulation against the total = 0.877).Preference and adaptation for mountain stream habitat might explain the past isolation andsubsequent divergence of Pseudobarbus phlegethon populations between these two rivers.Barbus calidus was genetically homogenous over its distribution (Nei's unbiased geneticdistance = 0-0.009; F-statistic for subpopulation against the total = 0.135). It is proposed thatBarbus calidus is better adapted than Pseudobarbus phlegethon to disperse through the mainstream. The genetic divergence between Barbus calidus and Barbus erubescens (Nei'sunbiased genetic distance = 0.063) is characteristic of conspecific populations. There seemsto have been a loss of genetic diversity in redfin populations of the Doring River tributaries(heterozygosity = 0-0.01), compared to the redfin populations of the Olifants River tributaries(heterozygosity = 0.01- 0.04). From phylogenetic analysis ofallozyme characters, it seems asif the serrated tetraploid barbs from South Africa is the sister-group of Pseudobarbus, whilstBarbus anoplus was rejected as a sister-species for the latter. Barbus calidus, Barbuserubescens and Barbus serra were found to be closely related to each other. The ancestor ofthe redfins seems to have been present in the Cape Fold Mountains since at least the late-Tertiary. On the basis of distributional and allozyme information, conservation unitsreflecting historical divergence, historical gene flow and current gene flow were identified asEvolutionarily Significant Units, Historical Management Units and Current ManagementUnits respectively. The Olifants and Doring River populations of Pseudobarbus phlegethonshould be recognized as two distinct Evolutionarily Significant Units. Barbus calidus forms aseparate Evolutionarily Significant Unit from Barbus erubescens. Twelve HistoricalManagement Units and nineteen Current Management Units were recognized for redfins ofthe Olifants River System. The size of Current Management Units should be expanded andsecured to prevent loss of genetic diversity. It is recommended that a recovery program ofredfins should establish new populations of at least Barbus erubescens and Doring RiverPseudobarbus phlegethon Evolutionarily Significant Unit. Centrarchids should beirradicated, as they are the main reason for the decline of red fins.

[发布机构] Stellenbosch University