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Biological development, i.e., the process by which a highly complicated multi-tissular organism develops from an aparently uniform homogeneous cell, the fertilized oocyte, remains as mysterious today as the first time that it was observed by the curious eye of a man. Even though, especially in recent times by the joint effort of the genetical, cell biological and molecular approaches (Sambroek, 1985, Wilkins, 1986, Davidson, 1987), our knowledge of the details of embryonic development has been greatly extended, we still lack a true understanding of how this process occurs. In fact a fascinating story of complex tempo-spatial patterns of gene activity is currently being described (for instance, Lawrence et al, 1987), but its mechanistically linkage to the phenoytpe produced, the normal larva and adult, remains basically untackled.

Although early in this century the study of cell metabolism in developmental systems was a major concern (e.g,, see Needham, 1931, 1942; Willier et al., 1955), the current surge of molecular biology has shifted the interest of the majority of developmental biochemists to the study of the regulation of gene expression during development. Without discussing the obvious conceptual and historical reasons behind such change, this virtual elimination of a formerly fashionable subject has left many unanswered questions, if not for other reasons, as a consequence of the limited technical and conceptual potentiality of biochemistry half a century ago.

Previous work by members of our group [1,2,3] suggested that in Artemia early development a considerable proportion of the mitochondria were stored in an inactive state in the yolk granules from which they were released during the developmental process, becoming active as power generators in the nauplii. In order to test conclusively this idea, we turned to the mitochondrial DNA as we thought that it could provide the best marker for the presence and quantitation of these organelles, independent of their functional status. Very little was known about this interesting informational molecule in Artemia [4], but in other systems as distant as sea urchin, amphibians and mammals [5,6,7] it is known that it is basically synthesized during oogenesis, the mature oocyte containing all the mitochondria necessary for early embryogenesis.