On the meaning of chance in biology

Chance has somewhat different meanings in different contexts, and can be taken to be either ontological (as in quantum indeterminacy) or epistemological (as in stochastic uncertainty). Here I argue that, whether or not it stems from physical indeterminacy, chance is a fundamental biological reality that is meaningless outside the context of knowledge. To say that something happened by chance means that it did not happen by design. This of course is a cornerstone of Darwin's theory of evolution: random undirected variation is the creative wellspring upon which natural selection acts to sculpt the functional form (and hence apparent design) of organisms. In his essay Chance & Necessity, Jacques Monod argued that an intellectually honest commitment to objectivity requires that we accord chance a central role in an otherwise mechanistic biology, and suggested that doing so may well place the origin of life outside the realm of scientific tractability. While that may be true, ongoing research on the origin of life problem suggests that abiogenesis may have been possible, and perhaps even probable, under the conditions that existed on primordial earth. Following others, I argue that the world should be viewed as causally open, i.e. primordially indeterminate or vague. Accordingly, chance ought to be the default scientific explanation for origination, a universal 'null hypothesis' to be assumed until disproven. In this framework, creation of anything new manifests freedom (allowing for chance), and causation manifests constraint, the developmental emergence of which establishes the space of possibilities that may by chance be realized.

Mitochondria and metazoan epigenesis

In eukaryotes, mitochondrial activity controls ATP production, calcium dynamics, and redox state, thereby establishing physiological parameters governing the transduction of biochemical signals that regulate nuclear gene expression. However, these activities are commonly assumed to fulfill a 'housekeeping' function: necessary for life, but an epiphenomenon devoid of causal agency in the developmental flow of genetic information. Moreover, it is difficult to perturb mitochondrial function without generally affecting cell viability. For these reasons little is known about the extent of mitochondrial influence on gene activity in early development. Recent discoveries pertaining to the redox regulation of key developmental signaling systems together with the fact that mitochondria are often asymmetrically distributed in animal embryos suggests that they may contribute spatial information underlying differential specification of cell fate. In many cases such asymmetries correlate with localization of genetic determinants (i.e., mRNAs or proteins), particularly in embryos that rely heavily on cell-autonomous means of cell fate specification. In such embryos the localized genetic determinants play a dominant role, and any developmental information contributed by the mitochondria themselves is likely to be less obvious and more difficult to isolate experimentally. Hence, 'regulative' embryos that make more extensive use of conditional cell fate specification are better suited to experimental investigation of mitochondrial impacts on developmental gene regulation. Recent studies of the sea urchin embryo, which is a paradigmatic example of such a system, suggest that anisotropic distribution of mitochondria provides a source gradient of spatial information that directs epigenetic specification of the secondary axis via Nodal-Lefty signaling.

Mitochondria, redox signaling and axis specification in metazoan embryos

Mitochondria are not only the major energy generators of the eukaryotic cell but they are also sources of signals that control gene expression and cell fate. While mitochondria are often asymmetrically distributed in early embryos, little is known about how they contribute to axial patterning. Here we review studies of mitochondrial distribution in metazoan eggs and embryos and the mechanisms of redox signaling, and speculate on the role that mitochondrial anisotropies might play in the developmental specification of cell fate during embryogenesis of sea urchins and other animals.