Reproductive isolation and patterns of genetic differentiation in a cryptic butterfly species complex

Molecular studies of natural populations are often designed to detect and categorize hidden layers of cryptic diversity, and an emerging pattern suggests that cryptic species are more common and more widely distributed than previously thought. However, these studies are often decoupled from ecological and behavioural studies of species divergence. Thus, the mechanisms by which the cryptic diversity is distributed and maintained across large spatial scales are often unknown. In 1988, it was discovered that the common Eurasian Wood White butterfly consisted of two species (Leptidea sinapis and Leptidea reali), and the pair became an emerging model for the study of speciation and chromosomal evolution. In 2011, the existence of a third cryptic species (Leptidea juvernica) was proposed. This unexpected discovery raises questions about the mechanisms preventing gene flow and about the potential existence of additional species hidden in the complex. Here, we compare patterns of genetic divergence across western Eurasia in an extensive data set of mitochondrial and nuclear DNA sequences with behavioural data on inter- and intraspecific reproductive isolation in courtship experiments. We show that three species exist in accordance with both the phylogenetic and biological species concepts and that additional hidden diversity is unlikely to occur in Europe. The Leptidea species are now the best studied cryptic complex of butterflies in Europe and a promising model system for understanding the formation of cryptic species and the roles of local processes, colonization patterns and heterospecific interactions for ecological and evolutionary divergence.

[Dynamics of chromosome number evolution in the Agrodiaetus phyllis species complex (Insecta: Lepidoptera)]

We employed phylogenetic comparative method to study karyotype evolution in the Agrodiaetus phyllis species complex in which haploid chromosome numbers vary greatly ranging from 10 to 134. We have found that different phylogenetic lineages of the group have different rates of chromosome number changes. Chromosome numbers in the complex posses phylogenetic signal, and their evolutionary transformation is difficult to explain in terms of punctual and gradual evolution.

[Dobzhansky's rule and reinforcement of pre-zygotic reproductive isolation in zones of secondary contact]

It is well known that closely related sympatric species are usually more different in characters involved in species recognition (e.g., in visual and acoustic signals) than allopatric species of the same evolutionary age. In this article I call this phenomenon Dobzhansky's rule in accordance with the name of the scientist who first discovered it. There are two alternative explanations for this pattern. Under hypothesis of reinforcement by Dobzhansky, these species-specific differences evolve in situ, exactly in zone of overlap between two populations. Under hypothesis of differential fusion by Templeton, the differences originate in geographically separated regions, and only those populations that have evolved such differences can persist in secondary sympatry. These evolutionary scenarios are significantly different. The scenario by Dobzhansky is an essentially sympatric model, in which natural selection reinforces pre-zygotic isolation between divergent populations by selecting against unfit hybrids. The scenario by Templeton is based on classic allopatric speciation model that consider the formation of reproductive isolation to be a by-product of divergent evolution. In this work we show that the sympatric distribution of sister taxa of Agrodiaetus butterflies strongly correlates with differences in male wing colour. We also use a new quantitative phylogenetic test to distinguish between the models by Dobzhansky and by Templeton and to demonstrate that the pattern observed is, most likely, the result of reinforcement.

[What gene and chromosomes say about the origin and evolution of insects and other arthropods]

At the turn of the 21st century, the use of molecular and molecular cytogenetic methods led to revolutionary advances in systematics of insects and other arthropods. Analysis of nuclear and mitochondrial genes, as well as investigation of structural rearrangements in the mitochondrial chromosome convincingly supported the Pancrustacea hypothesis, according to which insects originated directly from crustaceans, whereas myriapods are not closely related to them. The presence of the specific telomeric motif TTAGG confirmed the monophyletic origin of arthropods (Arthropoda) and the assignment of tongue worms (Pentastomida) to this type. Several different types of telomeric sequences have been found within the class of insects. Investigation of the molecular organization of these sequences may shed light on the relationships between the orders Diptera, Siphonaptera, and Mecoptera and on the origin of such enigmatic groups as the orders Strepsiptera, Zoraptera and suborder Coleorrhyncha.

[Molecular and cytogenetic approaches to species diagnostics, systematics, and phylogenetics]

Modern approaches to systematics and phylogenetics, based on statistical analysis of molecular data, allow to conduct diagnostics and objective delimitation of taxa, establish alliances, detect monophyletic groups, verify the naturalness of polytypic species, reveal cryptic evolution lines, retrace the history of nascency and dispersion of species and populations, and reconstruct phylogenies. Nevertheless, there are limitations on molecular data use due to impossibility of paleontological material examination. Chromosomal rearrangements combine the advantages of molecular and morphological characteristics with the unique patterns of their evolution. Conservative parts of genome may be synapomorphies of high-rank taxa while labile regions reliably mark evolutional events associated with speciation. New chromosomal rearrangements are being fixed rather quickly in homozygous state which allows to solve such problem of related species systematics as polymorphism of standard markers (especially molecular ones), often exceeding the level of inter-species differences. Thus, karyosystematics armed with such modern chromosome studying method as FISH, maintains and even enhances its importance as an instrument for solving taxonomic and phylogenetic problems.