11 Addresses *Department of Genetics, John Innes Institute, Colney Lane, Norwich NR4 7UH, UK; e-mail: enrico.coen@bbsrc.ac.uk † Department of Plant Sciences, The University of Oxford, South Parks Road, Oxford OX1 3RB, UK; e-mail: jane.langdale@plants.ox.ac.uk Current Opinion in Plant Biology 2001, 4:11–13 1369-5266/01/$ —see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations AS1 ASYMMETRIC LEAVES1 CLV CLAVATA FIS FERTILISATION-INDEPENDENT SEED PAN PERIANTHIA SEP SEPALLATA WUS WUSCHEL Like the Twelve Labours of Hercules, the twelve reviews presented in this issue are concerned with various chal- lenges. Many of them have a historical flavour, highlighting areas in which our current understanding of plant development may provide significant insights into plant evolution. They begin, as does the story of Hercules, with double fertilisation. According to Greek legend, Hercules’ mother slept with two partners in close succession. The first coupling, with the god Zeus, led to the birth of Hercules, whereas the sec- ond, with a mortal, led to the birth of Iphicles, the lesser known twin brother of Hercules. In contrast, double fertil- isation in flowering plants leads to two genetically identical products, differing only in the dosage of maternal genomes: two in the endosperm and one in the embryo. As with Hercules, one of the products, the embryo, is des- tined to eclipse the other through its greater achievements and exploits (although when it comes to the food we eat, endosperms have their day). Double fertilisation was originally thought to be a defin- ing feature of all angiosperms, although, as Friedman (pp 14–20) points out, whether it occurs in the newly defined basal angiosperms, such as Amborella, has yet to be determined. How did double fertilisation arise? Recent studies in a broad range of angiosperms have revealed some features that are common to both endosperm and embryo development. Both undergo an initial unequal partition of the first cell followed by dif- ferential patterns of development of the two poles. This suggests that endosperm may have arisen from a super- numerary embryo that adopted a supporting role for its twin. So long as the twins are genetically identical, there is no conflict between genomes here — the glory of one redounds to the other. Genome conflict does rear its head, however, in the review on imprinting by Grossniklaus et al. (pp 21–27). Following fertilisation there is a potential conflict of interest between father and mother in the allocation of resources to offspring. It is in the mother’s interest to allocate her resources equally to all offspring, while the father’s interest is to divert as many of the mother’s resources as possible to the offspring that he himself has fathered. There is, therefore, a selec- tive pressure on the mother to retain control of early development thereby counteracting the potential greed of the paternal genome. Maternal control has been described for the FERTILISA- TION-INDEPENDENT SEED (FIS) class of genes, which encode proteins implicated in chromatin structure and gene regulation. Seeds derived from female gametophytes that carry a fis mutation abort irrespective of whether the paternal allele is mutant or wild-type. In some cases, this has been shown to reflect imprinting — paternal alleles are not expressed during early seed development whereas maternal alleles are. Several studies using methylation- deficient mutants have implicated DNA methylation as the basis of this imprint, although the FIS genes them- selves are unlikely to be the direct targets of methylation. Hercules encountered a slightly more extreme form of maternal control when he met the Amazons, a tribe of female warriors who broke the limbs of their infant boys in an attempt to control their behaviour later in life. A vital step in early plant development is the establishment and maintenance of a small group of stem cells in the shoot meristem. As described by Clark (pp 28–32), two groups of genes that play antagonistic roles in this process are WUSCHEL (WUS) and CLAVATA (CLV). WUS is expressed in a few L3 cells just below the apex and encodes a homeo- domain-containing protein required for maintaining stem cell activity in the overlying L1/L2 cells. It is thought that CLV genes act by curbing excessive WUS expression through transmission of a signal (CLV3) from stem cells in the L1 and L2 to a receptor complex (i.e. CLV1–CLV2) in the underlying L3 cells. This then leads to downregulation of WUS. Appropriately enough for our present theme, clavata means ‘shaped like a club’, an implement that Hercules was fond of using to beat his many opponents into submission. In the case of CLV–WUS, however, the interac- tion is not about attaining supremacy but about maintaining a balance that results in a well defined domain of stem cells within a continually proliferating apex. Sussex and Kerk (pp 33–37) discuss the importance of meristems from another viewpoint: the evolution of diverse Growth and development Of myths and meristems Editorial overview Enrico Coen* and Jane A Langdale †