The Evolution of Unisexuality in Calligrapha Leaf Beetles: Molecular and Ecological Insights on Multiple Origins via Interspecific Hybridization

Abstract

Abstract. Interspecific hybridization is a well-established cause of unisexual origins in vertebrates. This mechanism is also suspected in other apomictic taxa, but compelling evidence is rare. Here, we evaluate this mechanism and other hypotheses for the evolutionary origins of unisexuality through an investigation of Calligrapha leaf beetles. This group provides an intriguing subject for studies of unisexual evolution because it presents a rare insect example of multiple apomictic thelytokous species within a primarily bisexual genus. To investigate unisexual evolution, this study conducts the first molecular systematic analysis of Calligrapha. This involved the collection and analysis of about 3000 bp of DNA sequences-representing RNA and protein-coding loci from mitochondrial and nuclear genomes-from 54 specimens of 25 Calligrapha species, including four unisexual tetraploid taxa. Phylogenetic and molecular clock analyses indicated independent and single evolutionary origins of each of these unisexual species during the Pleistocene. Significant phylogenetic incongruence was detected between mitochondrial and nuclear datasets and found to be especially associated with the asexual taxa. This pattern is expected when unisexual lineages arise via interspecific hybridization and thus represent genetic mosaics that possess certain nuclear alleles from the paternal species lineage and mitochondrial DNA (mtDNA) alleles from the maternal parent. Analyzing the mtDNA and nuclear relatedness of unisexuals with corresponding haplotypes of bisexual Calligrapha species allowed the putative identification of these maternal and paternal species lineages for each unisexual species. Strong phenotypic similarities between unisexual taxa and their paternal parent species supported a model that involves both backcrosses of interspecific hybrids with a paternal parent and unreduced gametes. This model accounts for the origins of apomixis, polyploidy, and an overrepresentation of paternal nuclear alleles (and associated phenotypes) in unisexuals. This model is also consistent with the tetraploid karyotypes of unisexual Calligrapha, in which three sets of chromosomes (of presumed paternal ancestry) are quite morphologically homogeneous compared to the fourth. Especially intriguing was a consistent association of unisexual species with the host plant of the paternal parent but never with the maternal host. The statistical implausibility of these patterns occurring by chance further supports our inference of parental species. Moreover, it points to a potentially critical role for host-association in the formation and preservation of unisexual lineages. These findings suggest that ecological factors are critical for the diversification of unisexual as well as bisexual taxa and thus point out new research directions in the area of ecological speciation.

Nonrandom Patterns of Genetic Admixture Expose the Complex Historical Hybrid Origin of Unisexual Leaf Beetle Species in the GenusCalligrapha

“…Bisexual species were represented by one external outgroup considered for phylogenetic tests (C. dislocata; see below) and 13 of 18 species belonging to the clade where unisexual taxa originated, including all potential parental evolutionary lineages (Gó mez-Zurita et al 2006).…”

“…Yet in Calligrapha, the transition to unisexuality occurred many times independently, which makes it a very interesting evolutionary oddity, covered in the classical treatises on the evolution of sex (e.g., Maynard Smith 1978;Bell 1982). Unisexuality in Calligrapha has been unequivocally associated with tetraploidy (Robertson 1966), which likely arose via allotetraploidy (Gó mez- Zurita et al 2006). This, together with phylogenetic reasoning, suggests their hybrid origin.…”

“…Hybridization, bringing together genomic elements resulting from different evolutionary histories, is expected to result in incongruent phylogenetic signals from independent genetic markers (Brower et al 1996). Thus, several studies trying to unravel the hybrid origins of unisexuality have investigated this type of incongruence between mtDNA and nuclear data (e.g., Murphy et al 2000;Morgan-Richards and Trewick 2005;Gó mez-Zurita et al 2006). Nevertheless, phylogenetic incongruence may be caused by processes other than hybridization (Brower et al 1996;Funk and Omland 2003;Toews and Brelsford 2012), and this approach is rarely conclusive relative to hybrid origins.…”

“…Here we use this approach to test the hypothesis of hybrid origins of unisexual species or their ancestors in the leaf beetle genus Calligrapha, which was originally formulated based on incongruent phylogenetic patterns between mtDNA and nuclear phylogenies (Gó mez-Zurita et al 2006). Calligrapha is a species-rich New World beetle genus with nearly 40 species distributed in North America, including up to six known obligate unisexual taxa and one reported as facultative (Brown 1945;Robertson 1966;Gó -mez-Zurita et al 2004).…”

Origin of the complex karyotype of the polyploid parthenogenetic grasshopper Saga pedo (Orthoptera: Tettigoniidae)

“…Recent molecular studies on Coleoptera have revealed the complexity of the problem. It appears that, in a given species, both parthenogenesis and polyploidization can occur repeatedly and independently (Gómez-Zurita et al, 2006). As concluded by White & Contreras (1981), after an extensive study of the other parthenogenetic grasshopper Warramaba virgo "it is unlikely that the above questions can be answered in any simple universally valid manner".…”