Figure 1 a-d from Nozaki et al. 2018. Light microscopy of asexual spheroids in Taiwanese strains of Volvox carteri f. nagariensis. a Surface view of a spheroid showing undivided gonidia (G). 2016‐tw‐nuk‐6‐1. b Optical section of a spheroid in (a) with gonidia (G). c Surface view of spheroid. Note no cytoplasmic bridges between somatic cells. 2016‐tw‐nuk‐6‐1. d Surface view of spheroid showing individual sheaths of the gelatinous matrix. Stained with methylene blue. 2016‐tw‐nuk‐8‐1. e Optical section of gonidium. 2016‐tw‐nuk‐6‐1. f, g Pre‐inversion plakea or embryo (E) showing gonidia (G) of the next generation outside. 2016‐tw‐nuk‐8‐1.
Most of what we know about the developmental genetics of Volvox comes from the Eve strain of Volvox carteri forma nagariensis, which was collected by Richard Starr from Kobe, Japan in 1967. Eve is the strain that David Kirk and colleagues used for most of their experiments and from which most of the important developmental mutants are derived.
It’s natural, then, to think that Eve is representative of V. carteri f. nagariensis and that what’s true for Eve is generally true for this forma. Recent work from Hisayoshi Nozaki and colleagues shows that, at least in one respect, this is a bad approximation.
Dr. Nozaki’s team collected Volvox carteri f. nagariensis from several rice paddies in Taiwan, eventually isolating 33 strains (I’m just going to call them Volvox carteri from here out). There’s little doubt of their identity, since DNA sequences (internal transcribed spacer 2) from the newly collected strains are identical to those of Eve.
Volvox carteri is heterothallic, meaning that males and females are genetically distinct and it’s possible to establish purely male cultures and purely female cultures. Thirteen of these were female, nineteen male, and one was female by most indications but apparently lacked one female-specific gene (HMG1f).
Previous descriptions of Volvox carteri males, based on strains collected in Japan, report a 1:1 ratio of ratio of somatic cells to sperm packets, and this ratio has been treated as diagnostic (in other words, as a way to distinguish nagariensis from the other formas). However, among the newly collected strains, this ratio varied from 1:1 all the way to 50:1!
Figure 2 from Nozaki et al. 2018. Light microscopy of sexual reproduction in Taiwanese strains of Volvox carteri f. nagariensis. 2016‐tw‐nuk‐6‐1 (male) and 2016‐0609‐v‐1 (female). a Male spheroids (m) developing within parental spheroid. b–d Various ratios of somatic cells to sperm packets (S) in male spheroids. Note 4–12:1 ratio in (a and b), > 50:1 ratio in (c), and 1:1 ratio in (d). e Female spheroids with eggs (E). f Mature zygotes (Z) in female spheroid. g Optical section of mature zygote with reticulate wall.
This is a pretty extreme degree of variation. I’m sure there are other cases in which some trait varies 50-fold within a species, for example body weight in dogs. But this is not just within a species; it’s within a genotype. Individual spheroids within a Volvox strain are genetically identical. They’re basically identical twins (or tenthousandlets). It’s as if genetically identical corn kernels grew into plants from two inches to eight feet tall. It’s not the only example of extreme variability within strains of volvocine algae. I found that some strains of Pleodorina starrii evolved under selection for size included 4-, 8-, 16-, 32-, and 64-celled colonies….that’s the same genotype growing in the same tube at the same time.
Nozaki and colleagues attribute the differences between Japanese and Taiwanese Volvox carteri to geographical isolation:
Given that the Japanese and Taiwanese populations of V. carteri f. nagariensis grow in two geographically separated regions (temperate and subtropical, respectively), they can be presumed to be independent lineages derived from a common ancestor.
That’s a reasonable speculation, but it leaves some important questions. Understanding the mechanistic basis for the difference between the Japanese and Taiwanese strains would be fascinating. What are the genetic differences that cause one strain to produce such variable males while those of other strains are much less variable? And what mechanism causes the differences in somatic cell:sperm packet ratio within the Taiwanese strains? What’s going on in these colonies to cause such extreme variation within a genotype?
I’ve focused on just one aspect of this work; the paper goes into much more detail about the contents of the sex-determining regions in the male, female, and female-minus-HMG1f strains. The paper is open access, though, so if you want to dig into the genetics, check it out!
Stable links:
Herron, M.D., Ghimire, S., Vinikoor, C.R. and Michod, R.E. 2014. Fitness trade-offs and developmental constraints in the evolution of soma: An experimental study in a volvocine alga. Evol. Ecol. Res., 16: 203–221. http://www.evolutionary-ecology.com/abstracts/v16/2917.html
Nozaki, H., Ueki, N., Takusagawa, M., Yamashita, S., Misumi, O., Matsuzaki, R., et al. 2018. Morphology, taxonomy and mating-type loci in natural populations of Volvox carteri in Taiwan. Bot. Stud., 59: 10. DOI: 10.1186/s40529-018-0227-9
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