The Consequences of Long-Term Asexuality
While there is a rich body of theory supporting various explanations for the prevalence of sex, what has been lacking is an experimental model of long-term asexual evolution–  an exception to the general rule that the loss of sex leads rapidly to extinction. The class Bdelloidea, a group of more than 370 species of micro-invertebrates in the basal metazoan phylum Rotifera, may be such a system.
     Bdelloid rotifers are common aquatic invertebrates a few tenths of a millimeter long. Characterized by their ciliated head structure and bilateral ovaries, they have ganglia; muscles; digestive and secretory organs; photosensitive and tactile sensory organs; and structures for crawling, feeding, and swimming. Bdelloid eggs are produced from oocytes by two mitotic divisions, without chromosome pairing and without reduction in chromosome number, each oocyte yielding one egg and two polar bodies.  Measured values of the DNA content of bdelloid G1 oocyte nuclei range from approximately 500 to 2,000 Mbp, depending on species.

The bdelloid rotifer Macrotrachela quadricornifera.  The large oval is an embryo which will undergo direct development into a juvenile.  The inset shows the oocytes (collection of small cicles) in an ovary surrounded by the nuclei of the vitallarium (larger cicles) which provides nutrients to the developing embryo.  Oocytes will develop without chromosome reduction or pairing into an embryo, which will be a  clone of the mother.  A female produces 15-30 eggs during a 25-40 day lifespan under laboratory conditions.  Find out more about rotifers .

Pathogen Resistance in an Ancient Asexual
Sexual recombination allows host species to adapt rapidly to new threats from pathogens. If bdelloid rotifers evolved without sexual recombination, how have they adapted to new parasites and pathogens?  Do they possess a particularly effective innante immune system or a novel means of generating a variable immune response?
    Nothing is known about microbial infections of bdelloids, although bdelloids flourish in habitats that contain non-specific microbial pathogens such as Enterococcus, Legionella, and Salmonella.  We are investigating the bacterial flora found in natural populations of bdelloids to determine which microbial infections decrease fecundity and whether there is genetic variation in response to these infections, as well as whether specific microbes co-exist with bdelloids in mutually beneficial symbioisis. This project is supported by the Eppley Foundation for Research.

The Ecological and Evolutionary Dynamics of Sex
The paradox of sex is particularly striking in cyclically parthenogenetic species, where the majority of reproduction is asexual but environmental cues trigger episodic sexual reproduction.  Populations of cyclically parthenogenetic species readily lose the ability to undergo meiosis, becoming obligately asexual, yet both sexual and asexual reproduction are maintained at the species level.  This suggests a dynamic interaction between reproductive mode, fitness, and population ecology.  Of central importance to understanding this dynamic is the evolution and ecological role of the genes that determine the frequency and prevalence of sex in cyclically parthenogenetic populations.  Ultimately, these genes influence genomic diversity, rates of adaptation and speciation, and levels of biocomplexity in ecosystems.
    In collaboration with Dr. Terry Snell and others at Georgia Tech, we are isolating and characterizing the genes responsible for inducing mixis and for mate recognition in monogonont rotifers-- one of the largest, most successful groups of cyclical parthenogens.  The Snell laboratory has purified the Mixis Induction Protein and the Mate Recognition Protein from the monogonont B. plicatilis, and we have used the N-terminal sequence of MRP to isolate a candidate MRP gene.  We are currently isolating a gene corresponding to the N-terminus of MIP, while the Snell lab is working to eliminate mate recognition through RNAi using the candidate MRP gene.  This project, which is funded is funded the NSF Biocomplexity in the Environment Program, will employ a combination of biochemical, genetic, genomic, bioinformatic, and modeling approaches to investigate natural and laboratory populations that have lost or maintained sex under a variety of ecological conditions.  By tracking the evolution of these genes and their interaction with the genome and the environment we hope to understand how genotype and environment interact to regulate sex and determine its frequency in cyclical parthenogens.

The monogonont rotifer Brachionus plicatilus (right, carrying a developing egg) is facultatively sexual.  Reproduction is generally asexual (amictic).  When population densities reach a certain point, a small percentage (~20%) of females will produce daughters that will then produce eggs by meiosis.  The Snell lab has determined that the mixis inducing factor is a secreted peptide.  If unfertilized, a haploid egg produced by a mictic female will develop into a highly reduced, haploid male that will attempt to mate with a mictic female.  Males recognize a 28 kD peptide on the corona of the female called the Mate Recognition Protein; the Snell lab has isolated and characterized this protein and we have isolated the gene which encodes it.  Fertilized haploid eggs become diploid resting eggs capable of desiccation tolerance and over-wintering.   Resting eggs develop into amictic females.  The sexual cycle is not required for short-term evolutionary success and it is often lost in laboratory cultures. Find out more about rotifers