Struttura genetica ed eterogeneità ambientale nel nasello (Merluccius merluccius)
Author(s)
Despini, Massimo
Date Issued
February 12, 2009
Type
Doctoral Thesis
Abstract
In this study the genetic structure of the European hake (Merluccius merluccius) has been investigated to look inside the paleo-geographic history of the sea basins included in the range of this species (Mediterranean Sea and
Northeastern Atlantic Ocean).
Literature data show a differentiation between Atlantic populations (including those from the Alboran Sea) vs. Mediterranean in this species. This differentiation, highlighted by allozyme studies, is mainly due to two enzyme loci
(Gapdh and Gpi-2) having allele frequencies patterns with a clinal distribution,
significantly correlated with the values of both salinity and temperature at the surface and at -320m. From this observation stems the hypothesis that selection is acting on European hake gene pool, causing the genetic differentiation of Atlantic and Mediterranean stocks. This claims for a deeper analysis, based on the
comparison between the allozymes and a positively neutral marker (as mtDNA) to be use as a reference term for comparison.
In the last years an increasing bulk of data is showing that paleogeographic events have had a high relevance in moulding the present phylogeographic pattern in marine species too, implying that building up paleogeographic scenarios is important to understand how evolutionary processes work. It has been shown that a wide variety of patterns are presented by marine species: from high genetic differentiation between Atlantic and Mediterranean population to no difference at all. Moreover the patterns recorded can be due in a different extent to
evolutionary forces (selection, gene flow…) acting today and to the effects of paleoclimatic changes in the past.
Since there is an evidence of selection acting nowadays on the gene pool of the European hake, the work of this PhD thesis has been to verify the real role played by selective processes in determining the genetic structure of this species
compared to the signs of the past history (paleobiogeographic scenarios).
Partial sequences of the D-loop mitochondrial gene have been obtained from hake samples taken over the whole range of the species. The pattern of diversity distribution at this mt gene has been compared to allozyme data in the
aim of identifying out-layer loci with significantly different patterns, hence under
selection. The same mitochondrial data have been used to reconstruct a palogeographic scenario.
The obtained results have shown that using the neutral marker D-loop it is not possible to discriminate between Atlantic and Mediterranean populations of European hake, in spite there are some rare haplotypes present in the Mediterranean Sea only. The comparison between allozyme and mitochondrial data has evidenced that only using the whole nuclear data set it is possible to
distinguish between the two stocks. Indeed, when the two allozyme loci suspected to be under selection (Gapdh and Gpi-2) are removed from the data set, the pattern shown by the allozymes become similar to that of the D-loop, without any
geographic subdivision.
Mitochondrial data suggest that the European hake underwent a demographic expansion during the Riss glaciation (about 180000 years ago). This
expansion triggered migrations within the Mediterranean Sea and from the Atlantic Ocean to the Mediterranean but not from the Mediterranean to the Atlantic. During glacial stages it is known that the cold Atlantic waters creates a
marine current flowing into the Mediterranean Sea at the sea floor level through the Straits of Gibraltar. This water flow brought the so called “cold hosts” into the Mediterranean Sea, allowing its colonization by Atlantic fauna during glacial
periods. This scenario fits the European hake, except that when the Atlantic hakes “colonized” the Mediterranean thanks to the favourable climatic conditions of Riss glaciation, it was still inhabited by the Mediterranean strain.
The bulk of data produced allows concluding that the genetic structure of the European hake is the result of both the past history of the species, i.e. the sign of paleogeographic events, and of the evolutionary forces, i.e. selection acting
today by contrasting the homogenizing action of the gene flow at each generation.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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