Mitochondrial DNA Length Variation and Heteroplasmy in Natural Populations of the European Sea Bass, Dicentrarchus labrax

Abstract

Mitochondrial DNA length variation in DicentrurchusZubrux is the outcome of the simultaneous variation in the copy number of two tandem repeat arrays located in the D-loop region. The repeated sequences, named R17 and R48, are 17 and 48 bp long, respectively. On the basis of 209 individuals collected in eight Mediterranean localities, length variation and its partitioning between the two repeat arrays have been characterized and quantified by means of frequency distributions of repeat copy number and genetic diversity estimates. The median copy numbers are 4 and 11 for the R17 and R48 repeat arrays, respectively. For both repeat arrays, continuity in step variation and the occurrence of both frequent and rare arrangements was observed. More than 50% of the individuals are heteroplasmic for up to four variants. Within populations, heteroplasmy ranges between 33.3% and 70.0% of the individuals. A large proportion of the total gene diversity (29.1%) occurs within individuals and the greatest proportion (66.6%) is found between individuals within populations, while only 4.3% is due to variation between populations. Although the interpopulation component of the diversity is rather low, the significant differentiation of the two Eastern populations from all the other Western samples suggests the occurrence of a phylogeographic component of the genetic divergence between the Mediterranean populations, which is only recognizable on a large geographic scale.

Mitochondrial DNA variable number tandem repeats (VNTRs): utility and problems in molecular ecology

“…Moreover, there was no apparent PCR bias towards the shorter template repeat arrays. Skewed distributions have been reported for the short (17 bp) repeat, but not the long (48 bp) mtVNTR in the sea bass (Cesaroni et al 1997). A complicating factor with distributions skewed towards low repeat numbers is the repeat-size history.…”

“…Analysis of mtVNTR frequencies revealed significant differences between the two species in pure but not in hybrid populations. Cesaroni et al (1997) have employed PCR analysis of mtVNTR variation in populations of the sea bass Dicentrarchus labrax and were able to detect differences between eastern and western Mediterranean regions. Wilkinson & Chapman (1991) were able to differentiate between both eastern USA colonies and specific matrilines of the evening bat Nycticeius humeralis using copy number and frequency differences in a D-loop 81 bp repeat.…”

“…Some previous studies have suggested that biological processes, such as selection for smaller repeat size or directional mutation bias, may influence the distribution of VNTR size classes (Rand & Harrison 1986;Cesaroni et al 1997;Wierdl et al 1997). As such, skewed distributions to smaller size classes may potentially be either a biological process or a methodological laboratory artefact and should be noted early by investigators and the causes carefully determined by manipulation of amplification conditions.…”

“…D. White rhinoceros 10 bp repeat (Xu & çrnason 1997). E. Sea bass 48 bp repeat (Cesaroni et al 1997). cern might arise from poor-quality DNA templates, such as are often encountered when working with field-preserved specimens or museum samples. In such circumstances there is the potential for differential degradation of mtVNTRs with different lengths.…”

“…Rand & Harrison 1989;Wilkinson & Chapman 1991;çrnason & Rand 1992;Cesaroni et al 1997;Whipple et al 1998) assume an IAM, and seem more applicable to organelle sequences or mosaic repeats than mtVNTRs. Models based on the SMM may be more realistic for simple (not mosaic) mtVNTRs yet in terms of population differentiation one must then consider whether the amount of homoplasy (which is implicit in SMM) will influence the data set.…”