Evolutionary Genetics of Dictyostelids: Cryptic Species, Sociality and Sex
Kalla, Sara Edith
Strassmann, Joan E.
Doctor of Philosophy
Dictyostelium discoideum serves as an ideal system to study social evolution because of the social stage of its lifecycle, where individuals aggregate to build a multicellular structure. However, much of its basic biology remains unknown and this limits its utility. I used three separate projects to fill these gaps. In my first project, I examined how speciation and genetic diversity affects kin discrimination using a related dictyostelid, Polysphondylium violaceum . I sequenced the ribosomal DNA of 90 clones of P. violaceum and found that P. violaceum is split into several morphologically identical groups. When allowed to cooperate in pairwise mixes, I found that some clones cooperated with others in their group, but in mixes between groups, clones did not cooperate. For my second project, I looked at whether D. discoideum has sex in natural populations. While sex has been observed in laboratory clones of D. discoideum , it is unclear whether sex occurs in natural populations, and sex can influence the evolution of traits. I used a dataset of microsatellites in 24 clones of D. discoideum to look for a decrease in linkage disequilibrium as a molecular sign of sex. Linkage disequilibrium is higher between physically close loci than between loci on different chromosomes. From this, I conclude that D. discoideum undergoes recombination in nature. Lastly, I used the genome sequence of D. discoideum to look at large scale patterns of evolution. Mutations tend to be biased towards A/T from G/C so, on average, mutations should lower the nucleotide content of sequences. The removal of these mutations, purifying selection, should preserve nucleotide content. I used the genomes of D. discoideum and Plasmodium falciparum identify classes of sequences that should be under different amounts of purifying selection and compared their nucleotide contents. In all cases, those sequences under more purifying selection had higher GC contents than sequences under less purifying selection. Looking at relative nucleotide content may thus serve as an indicator purifying selection. These three studies add insight on how cooperation works in dictyostelids as well as adding an understanding of how traits, social and otherwise, would evolve in this system.
Biological sciences; Dictyostelids; Cryptic species; Speciation; Evolution & development