Biology

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Plasticity and regeneration of gonads in the annelid Pristina leidyi
(Springer Nature, 2016-10-04) Özpolat, B. Duygu; Sloane, Emily S.; Zattara, Eduardo E.; Bely, Alexandra E.
Gonads are specialized gamete-producing structures that, despite their functional importance, are generated by diverse mechanisms across groups of animals and can be among the most plastic organs of the body. Annelids, the segmented worms, are a group in which gonads have been documented to be plastic and to be able to regenerate, but little is known about what factors influence gonad development or how these structures regenerate. In this study, we aimed to identify factors that influence the presence and size of gonads and to investigate gonad regeneration in the small asexually reproducing annelid, Pristina leidyi. We found that gonad presence and size in asexual adult P. leidyi are highly variable across individuals and identified several factors that influence these structures. An extrinsic factor, food availability, and two intrinsic factors, individual age and parental age, strongly influence the presence and size of gonads in P. leidyi. We also found that following head amputation in this species, gonads can develop by morphallactic regeneration in previously non-gonadal segments. We also identified a sexually mature individual from our laboratory culture that demonstrates that, although our laboratory strain reproduces only asexually, it retains the potential to become fully sexual. Our findings demonstrate that gonads in P. leidyi display high phenotypic plasticity and flexibility with respect to their presence, their size, and the segments in which they can form. Considering our findings along with relevant data from other species, we find that, as a group, clitellate annelids can form gonads in at least four different contexts: post-starvation refeeding, fission, morphallactic regeneration, and epimorphic regeneration. This group is thus particularly useful for investigating the mechanisms involved in gonad formation and the evolution of post-embryonic phenotypic plasticity.
Transcriptome characterization via 454 pyrosequencing of the annelid Pristina leidyi, an emerging model for studying the evolution of regeneration
(2012-06-29) Nyberg, Kevin G.; Conte, Matthew A.; Kostyun, Jamie L.; Forde, Alison; Bely, Alexandra E.
Background: The naid annelids contain a number of species that vary in their ability to regenerate lost body parts, making them excellent candidates for evolution of regeneration studies. However, scant sequence data exists to facilitate such studies. We constructed a cDNA library from the naid Pristina leidyi, a species that is highly regenerative and also reproduces asexually by fission, using material from a range of regeneration and fission stages for our library. We then sequenced the transcriptome of P. leidyi using 454 technology. Results: 454 sequencing produced 1,550,174 reads with an average read length of 376 nucleotides. Assembly of 454 sequence reads resulted in 64,522 isogroups and 46,679 singletons for a total of 111,201 unigenes in this transcriptome. We estimate that over 95% of the transcripts in our library are present in our transcriptome. 17.7% of isogroups had significant BLAST hits to the UniProt database and these include putative homologs of a number of genes relevant to regeneration research. Although many sequences are incomplete, the mean sequence length of transcripts (isotigs) is 707 nucleotides. Thus, many sequences are large enough to be immediately useful for downstream applications such as gene expression analyses. Using in situ hybridization, we show that two Wnt/β-catenin pathway genes (homologs of frizzled and β-catenin) present in our transcriptome are expressed in the regeneration blastema of P. leidyi, demonstrating the usefulness of this resource for regeneration research. Conclusions: 454 sequencing is a rapid and efficient approach for identifying large numbers of genes in an organism that lacks a sequenced genome. This transcriptome dataset will be a valuable resource for molecular analyses of regeneration in P. leidyi and will serve as a starting point for comparisons to non-regenerating naids. It also contributes significantly to the still limited genomic resources available for annelids and lophotrochozoans more generally.
Breaking the A-P axis: Evolution of diverse asexual reproduction strategies in Convolutriloba acoels
(2009) Sikes, James M.; Bely, Alexandra E.; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The defining characteristic of the Bilateria is the presence of a distinct head end and tail end, which defines the anterior-posterior (A-P) axis, a feature that is established during embryogenesis and generally remains unaltered during the lifetime of an organism. While a few bilaterians have evolved asexual reproduction strategies that allow them to subdivide the A-P axis, acoels in the genus Convolutriloba have an unparalleled ability to alter the A-P axis during modes of transverse fission, longitudinal fission, and reversed polarity budding. Convolutriloba acoels thus offer an exceptional opportunity to investigate the mechanisms that allow for the radical modification of an already established A-P body axis and to explore the evolution and development of diverse asexual reproduction strategies among related species. In this study, I reconstruct the evolutionary history of asexual reproduction in the Convolutriloba and compare the diverse modes of asexual reproduction at the level of body-wall musculature, nervous system development, and cell proliferation while also exploring the regenerative potentials of tissues across species with different modes of asexual reproduction. In addition, I further explore the unusual process of A-P axis reversal that occurs during reversed polarity budding in C. retrogemma through studies of body patterning and regeneration. The results of these analyses suggest that a rich developmental toolkit of regenerative abilities, including the ability to utilize both epimorphosis and morphallaxis, to regenerate all parts of its body even from a small fragment, and to produce bifurcated A-P axes were present in the ancestor of the Convolutriloba allowing for the evolution of A-P axis modifications unlike any other bilaterian group. This toolkit along with the evolution of a seemingly unpatterned zone of tissue within the body of C. retrogemma capable of generating new anterior axes appear to have allowed this species to evolve the ability to form reversed A-P axes during budding.

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