The monarch butterfly, Danaus plexippus, is famous for its spectacular annual migration across North America, recent worldwide dispersal, and orange warning coloration. Despite decades of study and broad public interest, we know little about the genetic basis of these hallmark traits. By sequencing 101 monarch genomes from around the globe, we uncover the history of the monarch's evolutionary origin and global dispersal, characterize the genes and pathways associated with migratory behavior, and identify the discrete genetic basis of warning coloration. The results rewrite our understanding of this classic system, showing that D. plexippus was ancestrally migratory and dispersed out of North America to occupy its broad distribution. We find the strongest signatures of selection associated with migration center on flight muscle function, resulting in greater flight efficiency among migratory monarchs, and that variation in monarch warning coloration is controlled by a single myosin gene not previously implicated in insect pigmentation.

Range expansions can result in founder effects, increasing genetic differentiation between expanding populations and reducing genetic diversity along the expansion front. However, few studies have addressed these effects in long-distance migratory species, for which high dispersal ability might counter the effects of genetic drift. Monarchs (Danaus plexippus) are best known for undertaking a long-distance annual migration in North America, but have also dispersed around the world to form populations that do not migrate or travel only short distances. Here, we used microsatellite markers to assess genetic differentiation among 18 monarch populations and to determine worldwide colonization routes. Our results indicate that North American monarch populations connected by land show limited differentiation, probably because of the monarch's ability to migrate long distances. Conversely, we found high genetic differentiation between populations separated by large bodies of water. Moreover, we show evidence for serial founder effects across the Pacific, suggesting stepwise dispersal from a North American origin. These findings demonstrate that genetic drift played a major role in shaping allele frequencies and created genetic differentiation among newly formed populations. Thus, range expansion can give rise to genetic differentiation and declines in genetic diversity, even in highly mobile species.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. At Doniana National Park (Spain) the lycaenid butterfly Cyaniris semiargus is univoltine and monophagous on Armeria velutina. After hatching, small larvae feed on flowers and undergo an 8-month diapause in the second or third instar, starting in late spring. 2. As a consequence of the highly permeable, sandy soil, the flowering phenology of A. velutina is constrained by a shortage of soil moisture. 3. Plant patches at altitudes over 17m a.s.l. (xerophytic) bloomed earlier than those at lower altitudes (hygrophytic), where plants may benefit from ground water. 4. The flight period of C. semiargus was similar in both types of patches. There was a noticeable synchronism between flowering phenology and oviposition period in hygrophytic patches, but not in xerophytic patches. 5. Egg density and prediapause larval survival, were significantly higher in hygrophytic patches (which can be considered as 'source' habiatas) than in xerophytic ones (which functions as 'sink' habitats). 6. The spatial variation in the intensity of the asymmetric interaction between C. semiargus and A. velutina is determined by asynchrony in flowering phenology among patches, which in turn depends on the water table depth.

Larvae and pupae of lycaenid butteflies are often associated with ants: this is usually a mutualism in which ants guard the lycaenids from natural enemies, and the lycaenid larvae and pupae provide sugars and amino acids for the ants. A possible consequence of the interaction is spatially correlated ant and lycaenid distributions, but the phenomenon is poorly documented. We examined the lycaenid Plebejus argus, which is tended by Lasius ants. Within habitat patches, P. argus eggs, larvae and pupae were all spatially associated with Lasius. On a larger scale, the densities of butterflies in different habitat patches and populations, and whether the butterfly was present or not, were correlated with Lasius ant densities. The association of P. argus with Lasius ants is consistent among populations, and occurs at several spatial scales. Other aspects of the ecology of P. argus are more variable.

A mechanism for co-operative load transport was detected in the seed-harvesting ant Messor barbarus. Workers co-operated sequentially, transferring the load from one to another and thus forming what might be termed a transport chain. Although most of the colony's workers could have transported loads unaided, co-operation was prompted by the appearance of high loading ratios in the vicinity of the food source. The first worker in such a chain tended to be small or medium-sized, with a high loading ratio, while the last was larger, and had a correspondingly lower loading ratio. This strategy was of major benefit to the colony, in that it considerably reduced the time required for transport to the nest.

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