The recent discovery that the unicellular green alga Oophila amblystomatis, invades embryonic tissues and cells of the salamander Ambystoma maculatum prompted us to investigate the growth and life history transformations of the algal symbionts in egg capsules. During embryonic development, symbionts were first detected microscopically as a cohesive population of swimming cells in the vicinity of the blastopore around embryonic stage 17. This population of cells grew and at embryonic stage 25, a fraction of the population began to affix to the inside of the egg capsule. Cells then underwent syngamy, lost flagella, and transformed into non-motile cells. We observed a linear increase in the accumulation of such capsule-associated cells from embryonic stage 25 to 40. The population of zoospores did not grow over this period and showed a declining trend between stage 39 and 40. We verified the population growth by measuring relative chlorophyll a content and also measured quantum yield (QY) of photosystem II (PS II) using pulse amplitude modulated (PAM) fluorometry. The population, but not the cell size, of non-motile capsule membrane-bound cells increased modestly during a one-month period after hatching, and continued to contain high levels of chlorophyll a and photosynthetic capacity. We conclude that O. amblystomatis undergoes a life history transition in egg capsules and speculate that many of these symbionts become zygotes, rather than invading the embryo.
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Pulse amplitude modulated fluorometry
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This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada through a Discovery Grant to CDB and a NSERC Research Tools and Instruments Grant to AGM. Ainslie Cogswell is thanked for finding and delivering the Mira River egg mass used in this study. David Garbary, two anonymous reviewers, and the editor are thanked for providing critical feedback.
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Representative images from capsular preparations, depicting the accumulation of capsule membrane-bound algae as a function of time. With the exception of cases in which the number of algae was sufficiently low to count manually, these images were subjected to automated counts (see Methods). Preparations are from (a) May 18 (b) May 22 (c) May 27 (d) May 30 (e) June 6. Scale bar = 0.5 mm (DOCX 24,246 kb)
The distributions of cell area, expressed as pixels2, for all preparations of capsular algae as a function of time. Each histogram represents area counts from a single image and a column of histograms represents five different capsules for a given date. The black vertical line represents the median bin (DOCX 383 kb)
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Bishop, C.D., Miller, A.G. Dynamics of the growth, life history transformation and photosynthetic capacity of Oophila amblystomatis (Chlorophyceae), a green algal symbiont associated with embryos of the northeastern yellow spotted salamander Ambystoma maculatum (Amphibia). Symbiosis 63, 47–57 (2014). https://doi.org/10.1007/s13199-014-0287-x
- Oophila amblystomatis
- Ambystoma maculatum