Genome evolution in parthenogenetic geckos of the Heteronotia binoei complex (Gekkonidae)

The Bynoe’s gecko (Heteronotia binoei) is a complex of chromosomally differentiated geckos that includes both sexual and parthenogenetic forms.  The parthenogenetic geckos arose via reciprocal hybridization events between two chromosomally distinct sexual forms - the “CA6” and “SM6” forms.  The “3N1” parthenogenetic form has a CA6 maternal ancestor, and therefore a CA6-type mitochondrial genome.  Similarly, the “3N2” parthenogenetic form has an SM6 maternal ancestor, and therefore an SM6-type mitochondrial genome.  Additionally, parthenogens are triploid with uneven genome dosages - “A” forms have a CA6/CA6/SM6 genome, while “B” forms have a CA6/SM6/SM6 genome (Moritz 1993).

Male CA6, Ninghan Station, Western Australia.

Heteronotia have a broad distribution; shown here are the distributions of the main chromosomal groups, H. spelea, H. planiceps, as well as the 3N1 and 3N2 parthenogenetic forms (dotted outlines).  The Carrarang Peninsula (*) is an important region because it is the potential site for the origin of the parthenogenetic lineages.  One component of my dissertation will be an investigation of the systematics and biogeography of Heteronotia using a multi-locus approach.  You can watch a movie of my paper presented at Evolution 2008 here (results subject to change as I include more critical data).

Evolutionary dynamics of tandem duplications in the mitochondrial genomes of parthenogenetic geckos (Heteronotia binoei)

The mitochondrial genomes of parthenogenetic Heteronotia have large, tandem duplications that encompass tRNA, rRNA, and protein coding genes, as well as the control region.  These duplications provide us with an opportunity to examine molecular evolutionary dynamics, including rare glimpses at intermediate steps of the duplication-random loss model of gene rearrangement (left), as well as mechanisms of gene duplication in mitochondrial genomes.  The general presence of duplications in the mitochondrial genomes is an interesting phenomenon, and I am currently thinking about hypotheses - perhaps associated with the absence of males - that can explain this pattern.  This research has been published in the December 2007 issue of Molecular Biology and Evolution.

Duplication-random loss model of nad5/nad6 gene rearrangement in 3N1 parthenogens. (A) rearranged and, (B) standard gene order.

modified from Strasburg and Kearney (2005)

We proposed a slipped-strand mispairing mechanism to explain the repeated origins of the tandem duplications in 3N1 parthenogens.  The heavy strand commences DNA synthesis at the D-loop (dark dotted line), displacing the old heavy strand in the process (solid dark).  The light strand begins at the origin of light strand synthesis (light dotted), but at some point dissociates (perhaps due to reduced interactions among components of the replication machinery due to the hybrid nature of the genome), reannealing and completing replication at some other location.  The end result is a large, tandem duplication.

Differential gene expression in parthenogenetic geckos (Heteronotia binoei; Gekkonidae)

Even though individuals in parthenogenetic lineages Heteronotia have identical sequences, their expression profiles may differ.  This provides a mechanism for parthenogens to exhibit phenotypic variation.  One component of my dissertation will quantify differential gene expression among the “A” and “B” clones from both 3N1 and 3N2 lineages.  There are several mechanisms that can lead to divergence in gene expression, including sequence divergence in regulatory regions, gene conversion, and other mechanisms of gene silencing.  Preliminary data already suggests that these mechanisms are in place.  Evidence for novel mutation (PFN3, ATPase-F) and gene conversion (SNRPD3, ATPase-F) are evident.

mRNA sequences from an SM6, 3N1, and CA6 individuals.  Shown here are polymorphic sites in the genes (sequence divergence for the gene are shown below).

Cytonuclear interactions in parthenogenetic geckos (Heteronotia binoei; Gekkonidae)

Physiology research from Michael Kearney’s research group (University of Melbourne) has found phenotypic variation in parthenogenetic Heteronotia that reflects interactions between the mitochondrial and nuclear genomes.  In collaboration with Kearney, I will examine the biochemical activity of the cytochrome c oxidase complex to quantify the variation in cytonuclear interactions in parthenogenetic lineages.  Preliminary data so far indicate that there does exist variation - first between 3N1 and 3N2, and then between “matched” (when two of three nuclear complements matches the mitochondrial background) and “mis-matched” genomes (when only one of three nuclear complements matches the mitochondrial background).

Preliminary cytochrome c oxidase activity.