A Research Proposal on the Origin Of Life. Closing Lecture given at the ISSOL Congress in Oaxaca,Mexico, on July 4, 2002

This paper describes some experiments the author would haveliked to carry out if he had started earlier in the origin-of-life field.The proposal is preceded by a hypothetical outline of the mainevents in the origin of life. According to this outline, the emergence oflife amounts to the transition between two kinds of chemistry: 1) cosmic chemistry, which is beginning to be understood and mostlikely provided the building blocks with which life was first constructed; and 2)biochemistry, the well-known set of enzyme-catalyzedmetabolic reactions that support all living organisms today and must havesupported the universal common ancestor, or LUCA, from which all known formsof life are derived. The pathway leading from one to the other of thosetwo chemistries may be divided into three stages, defined as the pre-RNA, RNA, and protein-DNA stages. A briefsummary of the events that may have occurred in these three stages and ofthe possible underlying mechanisms is given. It is emphasized that theseevents were chemical in nature and, especially, that theymust have prefigured present-day biochemical processes. Protometabolismand metabolism, it is argued, must have been congruent. With congruence as the underlying working hypothesis, threeproblems open to experimental investigation are considered: 1) the involvementof peptides and other multimers as catalysts of early biogenic chemistry;2) the participation of thioesters in primitive energy transactions;and 3) the influence of amino acids on the molecular selection of RNAmolecules.     

Clines: character number and the multivariate analysis of simple patterns of geographic variation

The stability of a clinal pattern geographic variation in relation to character number was investigated by the random selection of real characters. The characters used were selected from a set of 18 significant characters, drawn from four different character systems, and had a low within-group correlation. Principal component/coordinate analysis was used to elucidate the main pattern of geographic variation and the first principal coordinate was plotted against a geographical transect to illustrate this pattern. The existence of 'clinal' or 'monotically clinal' variation was assessed by the rank correlation between geographic position and coordinate score and the congruence between patterns of geographic variation was assessed by the correlation between the first principal coordinates of the appropriate analyses.
Two procedural models were used: A, the congruence between completely independent character sets based on 1–9 characters, and B, congruence between the 'total' analysis and those analyses based on 1–16 characters. The frequency of the types of pattern of geographic variation shown by these Model B analyses was also determined. Congruence is asymptotic in relation to character number and is described by various mathematical models, depending upon parameter and procedure. A broad pattern of clinal variation occurred in all analyses based on six or more characters and appears to be a stable feature of the racial affinities that is unlikely to be influenced by character choice. The details of the clinal pattern cannot be shown to be stable and may be influenced by the choice of character. This study had several findings in common with a previous study of categorical variation, e.g. the approximately hyperbolic relationship between character number and congruence in Model B, which may prove to be generally true in comparable situations.     

Molecules and morphology: Congruence of data inColumnea (Gesneriaceae)

The cladistic analysis and comparison of molecular and morphological data has been the source of much recent debate. In this study, independent analyses of molecular and morphological data fromColumnea L. sects.Pentadenia andStygnanthe (Gesneriaceae) are compared. Comparative methods include consensus, visually comparing trees from independent analyses and combined data analysis. Consensus methods provided little resolution. Comparison of trees obtained from the independent analyses revealed some differences although the trees are highly similar. However, a combined analysis found that the level of incongruence between the two data sets was low. The tree resulting from the combined data has aspects of both the morphological and molecular trees despite the larger number of molecular characters. In addition, the combined data tree has greater resolution than either of the two data sets singly, indicating that the two types of data are congruent, and complementary to each other.     

Selection for thermostability can lead to the emergence of mutational robustness in an RNA virus

Mutational robustness has important evolutionary implications, yet the mechanisms leading to its emergence remain poorly understood. One possibility is selection acting on a correlated trait, as for instance thermostability (plastogenetic congruence). Here, we examine the correlation between mutational robustness and thermostability in experimental populations of the RNA bacteriophage Qβ. Thermostable viruses evolved after only six serial passages in the presence of heat shocks, and genome sequencing suggested that thermostability can be conferred by several alternative mutations. To test whether thermostable viruses have increased mutational robustness, we performed additional passages in the presence of nitrous acid. Whereas in control lines this treatment produced the expected reduction in growth rate caused by the accumulation of deleterious mutations, thermostable viruses showed no such reduction, indicating that they are more resistant to mutagenesis. Our results suggest that selection for thermostability can lead to the emergence of mutational robustness driven by plastogenetic congruence. As temperature is a widespread selective pressure in nature, the mechanism described here may be relevant to the evolution of mutational robustness.     

Complex clines: the predictivity of complicated patterns of geographic variation portrayed by multivariate analysis

The stability of complex patterns of geographic variation was investigated by assessing the congruence between multivariate ordinations derived from randomly chosen real characters. Two series of populations were analysed representing two situations with complex patterns of geographic variation. The first, a ‘Eurasian’ series of populations, showed a strongly structured hierarchical pattern, the second, an ‘eastern’ series of populations, showed a more subtle complex pattern of smooth clines and steps. The characters were selected from a total of 81 (Eurasian) or 61 (eastern) within-population independent characters from six different systems. The congruence between ordinations of the geographical populations was measured by the rotational fit statistic, R². Three procedures were used to compare ordinations based on from two to up to 80 characters randomly chosen to give: A, completely independent character sets; B, subsets compared to the total set; and C, potentially overlapping sets. All three procedures showed that congruence between the ordinations was asymptotic in relation to character number. This relationship was described by one of two mathematical models (procedure B did not result in a hyperbolic model as found with simple patterns of geographic variation). Generally speaking, once a sufficient number of characters are used, the complex patterns of geographic variation are stable, reliable and predictive and not substantially influenced by character choice. The strongly structured hierarchical pattern required 15 or so characters to achieve reliability whilst the more subtle patterns required 20 or so characters. However, the addition of further characters does improve reliability in both cases. The greater percentage of variance portrayed by three-dimensional ordinations compared to two-dimensional ordinations is achieved at the cost of lower congruence when a sufficient number of characters are used. If case studies of geographic variation were to adopt these procedures (preferably using completely independent character sets; procedure A) the reliability of their results would be indicated).