Gómez-Zurita Lab
Herbivore Beetle Evolution
Animal Biodiversity and Evolution Programme
Institut de Biologia Evolutiva (CSIC-UPF)
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Evolution without sex
Research sponsored by
the Spanish Ministry of Science and Innovation (Plan Nacional 2008-2011).

Evolution without sex: Historical circumstances and
ecological implications of the hybrid origin of unisexual insects

Research team: Anabela Cardoso (research assistant, IBE), Daniel J. Funk (co-investigator, Vanderbilt University), Jesús Gómez-Zurita (IP, IBE), Adela González-Megías (co-investigator, Universidad de Granada), and Tinguaro Montelongo (PhD student, IBE).

Keywords: Phylogeny, phylogeography, molecular systematics, evolution, ecology, asexuality, interspecific hybridization, Coleoptera.

Unisexuality is paradoxically an unstable and rare reproductive mode in animals. Paradoxically because it is after all the most efficient, economic way to produce offspring. Its limitations to generate genetic variation able to cope and buffer environmental (biotic and abiotic) change supposedly explain the prevalence of sexual reproduction. Understanding an essential biological phenomenon such as sex thus benefits immensely from insight gained from the study of biological systems lacking it.

Unisexuality appears in Nature in a variety of forms and as the result of various mechanisms. Apomixis product of interespecific hybridisation is seemingly the most widespread in animals, but suitable models to investigate it in a comparative fashion are scarce. Within this context, we advocate in this proposal for the study of Calligrapha, a leaf beetle genus with several unisexual species hypothesised to be descendant of multiple independent interspecific hybridisation events. The comparative approach is granted with this model, providing several independent natural experiments of hybridisation (and subsequent unisexuality) and closely related lineages with alternative reproductive modes. 

Calligrapha (Coleoptera, Chrysomelidae) is a diverse genus of American beetles. With about one hundred species in the continent, it has the peculiarity to include some seven unisexual species in the northeastern range of its distribution, a very atypical reproductive strategy in chrysomelids. In the picture, the parthenogenetic C. apicalis (left), C. alnicola (middle) and C. virginea (right).

Our approach to investigate this system initially concentrates in hybridisation as focal process and its evolutionary implications. We will use molecular tools and phylogenetic and genetic approaches to verify this process to be responsible for unisexuality in Calligrapha. Incongruent phylogenetic patterns for nuclear and mitochondrial genes and bimodal allelic diversity in unisexual lineages will be seeked as evidence for hybridisation. We will also use multigene phylogeographies to analyse the historical circumstances which determined recurrent episodes of interspecific hybridisation, which shall be related to Pleistocene climate changes and associated range movements of bisexual species leading to secondary contacts and eventually interspecific matings. These genealogies will also inform about single or multiple origins of each unisexual phenotype (considered as species).

C. suturella is a unisexual taxon with a broad range in NE North America. Its origin has been unambiguously recognized as hybrid between females of C. philadelphica and males of C. multipunctata during the Pleistocene. A phylogeographic analysis including these three taxa shall provide a good deal of information about the history of this species' origin and that of its reproductive strategy. 

Finally, we will investigate the ecological implications that surround hybrid speciation and evolution, including trophic specialisation, female and larval performance, and mating preference. Hybridisation in Calligrapha involves parentals with different trophic selection; thus, unisexual hybrids inherit in theory a double trophic potentiality and broader niche, which can be interpreted as an ecologic advantage. For the same reason, different host selection by putative parentals, interspecific mating is in principle an unlikely outcome, unless parental and hybrid habitat tolerance are relaxed. Ultimately, we want to show that hybridisation is a crucial, almost unavoidable phenomenon in the evolution of this group of beetles, definitely of great scientific interest.