Terrestrial Palaeoecology and Global Change

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Chapter 8. Ecosystem evolution 323 c a d b Fig. 129. Fusion and retroconvergent modification of leaf segments in a Triassic peltasperm, Lepidopteris (Scytophyllum) vulgare (Krassilov, 1995c): (a, b, d) incomplete fusion of pinnules forming a lobed lamina, with interstitial pinnules incorporated in the web, betrayed by their persistent venation pattern (arrow); (c, e) further fusion producing an entire lamina, with pinnules, lost in fusion, still occasionally marked by fasciculate proximal veins; the distal veins appear dichotomous as in unmodified pinnules. e
324 Valentin A. Krassilov. Terrestrial Palaeoecology tern. The composite leaf blades now appear exactly as the simple pinnae of the prototype, except for the occasional “bushy” veins still bearing evidence of their secondary origin. Retroconvergence may seem an absurd circularity of morphological evolution when considered in terms of natural selection: an advantage of a derived composite structure, if any, is lost by returning back to the initial form. It is, however, meaningful as evidence of a genomic capability to hold an image (of initial pinnate leaf structure in the above case) and retrieve it after a series of morphological events. As in the neuronal system, there should be a short-term dispensable memory opening the gateways to a long-term memory based on structural changes at the molecular level (Waldrop, 1987). Insofar as the developmental patterns are extremely conservative, most developmental changes involve a short-term memory alone, rapidly resolving into a stereotype mode. Genetic assimilation means that, when repeatedly occurring over generations, the short-memory events might enter the long-term memory. This would result in parallelism of genomic trends during the development and over evolution, making the former a brief reiteration of the latter, as the “biogenetic law” compels. Directional evolution of genomes is invoked by a variety of epigenetic phenomena (Løevtrup, 1974; Jablonka & Lamb, 1995), including a non-random segregation of alleles (the miotic drive), the silencing of an allele through interaction with other alleles (paramutation: Brink, 1973), a selective usage of codons (Gouy & Gaultier, 1982; Duret & Mouchiroud, 1999; Urrutia & Hurst, 2001), etc. There is evidence of methylation being engaged in paramutagenicity (Walker & Panavas, 2001) as well as in a hormone-induced activation (Xu et al., 2001) or silencing of developmental genes (discussed in Quon & Berns, 2001). A significant correlation is found between gene expression and the selective usage of codons (Gouy & Gaultier, 1982; Duret & Mouchiroud, 1999; Urrutia & Hurst, 2001) indicating a possible role of differential gene functioning in a directional change of nucleotide composition. An exchange of nuclear and plasmid DNA (such as a high frequency escape of mitochondrial DNA to the nucleus in Saccharomyces: Thorsness & Fox, 1980) is another potential factor of directional genomic change. It is becoming increasingly clear that genome evolution is a result of multilevel regulation. At the level of post-translational modification, proteins with different functions are produced from a single coding sequence by the splicing mechanism. Recent studies reveal a high number of alternatively spliced genes, about 50% in the human genome (Brett et al., 2002). An environmentally induced change in a gene functioning (or a coordinated change of multiple gene functioning) may result in the loss of one or another splicing alternative (simultaneously in a number of functionally related spliced genes) with a major effect on the direction of genome evolution. At the level of sequential genomic structures, a combination of flexible repeated elements and the relatively constant unique elements not only complies with the two kinds of memory but also provides a mechanism of transforming one kind into the other. Contrary to a widely held view, genomic structures do not remain the same during devel-
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Terrestrial Palaeoecology and Global Change

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