Nuclear-mitochondrial conflict in the bladder campion Silene vulgaris. Populations of gynodiecious plants such as S. vulgaris consist of females (pictured here) and hermaphrodites, the latter producing both seeds and pollen [24,26]. Females are produced from hermaphrodites by mitochondrial variants that interfere with the development of the stamens, called cytoplasmic male sterility (CMS). Because they are primarily transmitted through ovules but not pollen, the mitochondria gain a fitness advantage from reducing allocation to male reproduction. CMS variants, however, are often subject to counter-evolutionary changes in nuclear-encoded genes that suppress their effects and restore male fertility. Photo courtesy of Doug Taylor, University of Virginia.

Worker policing in the wasp Dolichovespula saxonica [69]. Classic examples of altruism occur in the social insects, where workers forgo their own reproduction to raise the queen's offspring. Hamilton's inclusive fitness theory (also referred to as kin selection theory) showed that individuals might give up their personal fitness if it enhances that of their relatives (Box 1) [12,13]. Nevertheless, additional mechanisms can also be important for enforcing cooperation and preventing selfish behaviors [1,7,8,70]. Here, a worker that lays an egg (left) has that egg eaten by another worker (right). Photo courtesy of Kevin Foster, Harvard University.

Who's who in multicellularity. Self-nonself recognition in multicellular organisms that form chimeras. (a) Cryphonectria parasitica, the causal agent of chestnut blight. Hyphal fusion is restricted to strains that match at all vegetative incompatibility loci [71,72]. Incompatible reactions result in localized cell death and the formation of a barrage zone. Of the six pairings shown, only two (bottom right) are compatible. (b) In the marine tunicate Botryllus schlosseri, fusion or rejection is controlled by a highly polymorphic locus containing multiple immunoglobulin domains (FuHC) [41]. A single population can contain hundreds of different alleles. (c) In the social amoeba Dictyostelium discoideum, thousands of cells aggregate (far left) and subsequently develop into a fruiting body (far right). Cells of genetically different individuals can partially separate during multicellular development [42]. C. parasitica photo by Kent Loeffler, courtesy of the Department of Plant Pathology and Plant-Microbe Biology at Cornell University. B. schlosseri photo courtesy of Tony de Tomaso, University of California, Santa Barbara. D. discoideum, courtesy of Gerda Saxer, Rice University.