Hybrid zone origins, species boundaries, and the evolution of wing-pattern diversity in a polytypic species complex of North American admiral butterflies (Nymphalidae: Limenitis)

Hybrid zones present opportunities to study the effects of gene flow, selection, and recombination in natural populations and, thus, provide insights into the genetic and phenotypic changes that occur early in speciation. Here we investigate a hybrid zone between mimetic (Limenitis arthemis astyanax) and nonmimetic (Limenitis arthemis arthemis) populations of admiral butterflies using DNA sequence variation from mtDNA and seven nuclear gene loci. We find three distinct mitochondrial clades within this complex, and observe a strong overall concordance between wing-pattern phenotypes and mitochondrial variation. Nuclear gene genealogies, in contrast, revealed no evidence of exclusivity for either wing-pattern phenotype, suggesting incomplete barriers to gene exchange and/or insufficient time for lineage sorting. Coalescent simulations indicate that gene flow between these two subspecies is highly asymmetric, with the majority of migration occurring from mimetic into nonmimetic populations. Selective sweeps of alleles responsible for mimetic phenotypes may have occurred more than once when mimetic and nonmimetic Limenitis occurred together in the presence of the model (Battus philenor).

HOW BRIGHT AND HOW NASTY: EXPLAINING DIVERSITY IN WARNING SIGNAL STRENGTH

The conspicuous displays that warn predators of defenses carried by potential prey have been of interest to evolutionary biologists from the time of Wallace and Darwin to the present day. Although most studies implicitly assume that these "aposematic" warning signals simply indicate the presence of some repellent defense such as a toxin, it has been speculated that the intensity of the signal might reliably indicate the strength of defense so that, for example, the nastiest prey might "shout loudest" about their unprofitability. Recent phylogenetic and empirical studies of Dendrobatid frogs provide contradictory views, in one instance showing a positive correlation between toxin levels and conspicuousness, in another showing a breakdown of this relationship. In this paper we present an optimization model, which can potentially account for these divergent results. Our model locates the optimal values of defensive traits that are influenced by a range of costs and benefits. We show that optimal aposematic conspicuousness can be positively correlated with optimal prey toxicity, especially where population sizes and season lengths vary between species. In other cases, optimal aposematic conspicuousness may be negatively correlated with toxicity; this is especially the case when the marginal costs of aposematic displays vary between members of different populations. Finally, when displays incur no allocation costs there may be no single optimum value for aposematic conspicuousness, rather a large array of alternative forms of a display may have equal fitness.

Wing pattern evolution and the origins of mimicry among North American admiral butterflies (Nymphalidae: Limenitis)

The evolution of wing pattern diversity in butterflies has emerged as a model system for understanding the origins and maintenance of adaptive phenotypic novelty. Admiral butterflies (genus Limenitis) are an attractive system for studying wing pattern diversity because mimicry is common among the North American species and hybrid zones occur wherever mimetic and non-mimetic wing pattern races meet. However, the utility of this system has been limited because the evolutionary relationships among these butterflies remain unclear. Here I present a robust species-level phylogeny of Limenitis based on 1911 bp of two mitochondrial genes (COI and COII) and 904 bp of EF1-alpha for all five of the Nearctic species/wing pattern races, the majority of the Palearctic species, and three outgroup genera; Athyma, Moduza (Limenitidini), and Neptis (Limenitidinae: Neptini). Maximum-likelihood and Bayesian analyses indicate that the North American species are a well-supported, monophyletic lineage that is most closely related to the widespread, Palearctic, Poplar admiral (L. populi). Within North America, the Viceroy (L. archippus) is the basal lineage while the relationships among the remaining species are not well resolved. A combined maximum-likelihood analysis, however, indicates that the two western North America species (L. lorquini and L. weidemeyerii) are sister taxa and closely related to the wing pattern subspecies of the polytypic Limenitis arthemis species complex. These results are consistent with (1) an ancestral host-shift to Salicaceae by the common ancestor of the Poplar admiral and the Nearctic admiral lineage, (2) a single colonization of the Nearctic, and (3) a subsequent radiation of the North American forms leading to at least three independent origins of mimicry.

Fractal dimension in butterflies' wings: a novel approach to understanding wing patterns?

The geometrical complexity in the wings of several, taxonomically different butterflies, is analyzed in terms of their fractal dimension. Preliminary results provide some evidence on important questions about the (dis)similarity of the wing patterns in terms of their fractal dimension. The analysis is restricted to two groups which are widely used in the literature as typical examples of mimicry, and a small number of unrelated species, thus implying the consideration of only a fraction of the wing pattern diversity. The members of the first mimicry ring, composed by the species Danaus plexippus (better known as the monarch butterfly), and the two subspecies Basilarchia archippus obsoleta (or northern viceroy) and Basilarchia archippus hoffmanni (or tropical viceroy), are found to have a very similar value for the fractal dimension of their wing patterns, even though they do not look very similar at first sight. It is also found that the female of another species (Neophasia terlootii), which looks similar to the members of the previous group, does not share the same feature, while the Lycorea ilione albescens does share it. For the members of the second group of mimicry related butterflies, the Greta nero nero and the Hypoleria cassotis, it is shown that they also have very close values for the fractal dimension of their wing patterns. Finally, it is shown that other species, which apparently have very similar wing patterns, do not have the same fractal dimension. A possible, not completely tested hypothesis is then conjectured: the formation of groups by individuals whose wing patterns have an almost equal fractal dimension may be due to the fact that they do share the same developmental raw material, and that this common feature is posteriorly modified by natural selection, possibly through predation.