Origin and evolution of parthenogenetic genomes in lizards: current state and future directions

The atypical characteristics of parthenogenetic lizards offer a rare glimpse into the evolution of asexual vertebrate genomes, addressing the genetic consequences of 2 major hypotheses regarding the absence of sex: reduced potential for adaptation and the accumulation of deleterious mutations. As a consequence of their hybrid origin, parthenogenetic lizards exhibit admixed genomes that offer opportunities to study functional genomics and the disruption of coevolved gene complexes in a potentially perpetual hybrid background. The high heterozygosity also provides substantial signal to track instances of fundamental genomic processes, such as intergenomic recombination, transcriptional silencing, and mutation. The mitochondrial genomes of parthenogenetic lizards have unveiled evidence for both slipped-strand mispairing and unconventional initiation/termination of DNA replication as mechanisms generating large, tandem duplications that are fleeting in sexual animals, as well as a rare glimpse into the intermediate steps of the duplication-random loss model of mitochondrial gene rearrangement. Several important questions remain, for instance, how do polyploid, and in particular triploid, lineages solve issues of genome dosage? What are the molecular bases of meiosis and development that enable parthenogenesis? Expanding the synergy between natural history research and molecular biology promises to address these unanswered questions. Advances in methodology (such as genomic in situ hybridization) as well as high-throughput genome and transcriptome sequencing offer new opportunities to explore the persistent questions regarding asexual genome evolution with great precision.