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Molecular characterization of the mitochondrial control region of the milkfish, Chanos Chanos : utility and application to population genetic studies

Rachel June RAVAGO-GOTANCO student laureate

°1974 The Philippines
Master’s in molecular biology and biotechnology, University of the Philippines, 2001

Molecular characterization of the mitochondrial control region of the milkfish, Chanos Chanos : utility and application to population genetic studies

The global production of wild caught fish has levelled off, while the production of aquacultured fish keeps on increasing annually. Fish production in tropical coastal ecosystems can be very high, but unregulated fishing and damage to the habitat are having a negative impact. In addition, the biological basis of many economically important marine species remains poorly understood. There is a major shortage of reliable scientific information.
The milkfish is a medium sized fish, which lives in the water column and on the bottom and feeds on algae and small marine organisms. It is widely distributed across the tropical Indo-Pacific in coastal wetlands (estuaries and mangroves) and marine coastal waters. It sustains a very important aquaculture (more than 400,000 ton) and fishery. In The Philippines alone more than 800,000 people are employed in the milkfish production and processing. For a long time aquaculture has been hampered by a poorly controlled reproductive cycle, which necessitated the capture of wild larvae. Until now, artificial breeding has been implemented only for a short time. Therefore, cultured milkfish has not been domesticated yet. An important step in initiating domestication is the genetic characterisation of wild populations, so that strains of milkfish can be evaluated and selected for targeted selection.
The thesis of Rachel Ravago deals exactly with the genetic characterisation of wild milkfish populations, as the scientific literature was not unanimous on this issue. To that purpose, a small fragment of the cell genome (the DNA included in the mitochondria) has been extensively characterised, a new genetic marker has been developed and used for comparison among widely separated populations.
The study encompasses three related aspects. The first aspect deals with the genetic variation in the mitochondrial genome. Milkfish DNA shows extensive length variability and heteroplasmy due to the presence of a 41 bp tandemly repeated sequence. The results have been published in the internationally high ranking journal Marine Biotechnology. The second aspect evaluates the utility of this variation in characterising natural populations. As little variation could be detected between populations, it was not retained as a suitable marker. In a third and final aspect, the genetic structure was studied in natural populations in the Indo-Pacific, based on another marker, namely sequence variation of the mitochondrial control region. On a large scale, samples from the Philippines (Western Pacific), Hawaii (Eastern Pacific) and India (Indian Ocean) clearly differ from each other. The Philippine samples from Currimao (North Coast) differ from the Digos (South Coast) and nearby Bolinao (North Coast) samples. Hence, milkfish populations should be treated as distinct genetic units, which might also imply distinct life-history and ecophysiological characteristics.
It is the latter information which is of high relevance to the management of the fishery and the optimisation of aquaculture. As milkfish populations differ considerably across the Indo-Pacific, the fishing authorities should take this diversity into account. On the other hand, aquaculture can profit from an enhanced production by selecting wild strains with a high potential for growth and survival to disease. Ms. Ravago has made a splendid contribution to the sustainable use of the natural resources in her country and the Indo-Pacific, and to the protection of marine biodiversity.

report by Prof. Dr. Filip Volckaert, Laboratory of Aquatic Ecology, Katholieke Universiteit Leuven, Belgium