Cells, colonies, and clones: individuality in the volvocine algae


Biological Individuality

As I mentioned previously, I have a chapter in the newly published book Biological Individuality, Integrating Scientific, Philosophical, and Historical Perspectives. The chapter was actually written nearly five years ago, but things move more slowly in the philosophy world than that of biology. Finally, though, both the print and electronic versions are now available; here is the electronic version of my chapter. The book currently has no reviews on Amazon, so if you want to give it a read, yours could be the first. If you’re interested in current and historical views on individuality, there is a lot of good stuff in here, including contributions by Scott Lidgard & Lynn Nyhart, Beckett Sterner, Andrew Reynolds, Snait Gissis, Olivier Rieppel, Michael Osborne, Hannah Landecker, Ingo Brigandt, James Elwick, Scott Gilbert, and Alan Love & Ingo Brigandt.

My chapter is a consideration of what it means to be a biological individual, using the volvocine algae as a case study. Pretty much the intersection of my interests, and both common topics on Fierce Roller.

Unfortunately, the figures were converted to black and white. This makes sense for the print version, since color figures are expensive to print (and time-consuming, as I can attest from my days running a printing press). I don’t know why the electronic versions couldn’t have retained color figures, though. The black and white versions don’t lose any important information, but they do fail to capture the glorious beauty of the volvocine algae:

Figure 2.1 from Herron 2017.

Figure 2.1 from Herron 2017. Representative volvocine algae. A: Chlamydomonas reinhardtii, a unicell with two flagella at the anterior. B: Gonium pectorale, a flat or slightly curved plate of 8 to 32 cells (8 in this example), all oriented in the same direction (photos A and B by Deborah Shelton). C: Eudorina elegans, a spheroid with up to 32 undifferentiated cells (16 in this example). D: Pleodorina starrii, a partially differentiated spheroid with up to 64 cells (32 in this example). The small cells near the anterior pole (top) are terminally differentiated somatic cells specialized for motility; the larger cells perform both reproductive and motility functions. E: Volvox carteri, a spheroid with ca. 2000 small somatic cells arranged at the periphery and a handful of much larger reproductive cells (gonidia). F: Volvox tertius, a spheroid with ~1000 small somatic cells arranged at the periphery and a handful of much larger reproductive cells. The germ cells in this colony have begun to develop into daughter colonies, and one is in the process of inversion. G: Volvox barberi, a spheroid with ca. 30,000 small somatic cells arranged at the periphery and a handful of much larger reproductive cells. The germ cells in this colony have begun to develop into daughter colonies, and some are in the process of inversion. H. Volvox aureus, a spheroid with up to ~2000 small somatic cells arranged at the periphery and a handful of much larger reproductive cells.

I argue that the volvocine algae are a good model system for exploring ideas related to biological individuality:

The volvocine algae illustrate two of the classic problems plaguing discussions of individuality. First, a typical volvocine life cycle involves many rounds of asexual reproduction punctuated by occasional rounds of sexual reproduction (Fig. 2.2), and so many genetically identical colonies may descend from a single zygote. This is an instance of the ramet vs. genet problem: the genetically unique and (largest) genetically homogeneous units are the descendants of a given zygote (i.e., a genet; Sarukhán and Harper 1973), which may include a large number of physiologically discrete and autonomous colonies (i.e., ramets; Stout 1929)…The second problem is whether we should consider a colony of a given species to be a group of individuals (the cells) or an individual in its own right. This question has deep historical roots, as Ehrenberg, contrary to van Leewenhoek [sic] (1700) and Linnaeus (1758), considered a Volvox spheroid a colony of hundreds or thousands of individuals (Ehrenberg 1832).

Ah, crap. I misspelled van Leeuwenhoek, and it made it though all the many revisions to the final version. I wish, too, that I had expanded a bit on the historical applications of Volvox to the question of individuality, at least including examples from August Weismann and J. S. Huxley.

Figure 2.2 from Herron 2017.

Figure 2.2 from Herron 2017. Example of a volvocine life cycle (based on that of Eudorina). Each cell in a haploid asexual spheroid undergoes a series of rapid cell divisions early in development, eventually producing a juvenile spheroid, which is eventually released from the mother spheroid. In species with cellular differentiation, only the reproductive cells divide. Juveniles escape from the parental spheroid possessing all of the cells they will have as adults; continued growth occurs by increases in cell size and in the volume of extracellular matrix rather than by cell division. In isogamous species, cells differentiate into gametes of opposite mating types. In anisogamous species, cells differentiate into motile sperm packets or immotile eggs. In either case, fertilization results in a diploid zygote that eventually matures into a dormant, desiccation-resistant spore. Spores germinate through meiosis upon the return of optimal growth conditions.

I won’t rehash it here, but the chapter goes through the implications for individuality of Kirk’s 12 steps:

Figure 2.3 from Herron 2017.

Figure 2.3 from Herron 2017. Evolutionary relationships and estimated divergence times among volvocine algae. 1: incomplete cytokinesis; 2: partial inversion; 3: rotation of the basal bodies; 4: establishment of organismal polarity; 5: transformation of the cell wall into extracellular matrix; 6: genetic control of cell number; 7: complete inversion; 8: increased volume of extracellular matrix; 9: sterile somatic cells; 10: specialized germ cells; 11: asymmetric division; 12: bifurcated cell division program (steps 11 and 12 may have had 2 separate origins in the clade including V. africanus and V. carteri); 13: small gonidia, growth between divisions, and retention of cytoplasmic bridges in adult spheroids; 14: slow divisions and light-dependent divisions.

The central argument of the chapter is that if individuals are units of evolution, as most recent treatments consider them to be, then cells, colonies, and clones are all likely to have some degree of individuality under some conditions:

A number of authors concerned with individuality see individuals as the units of evolution. In the views of Godfrey-Smith (2009), Clarke (2012), and Michod and Nedelcu (2003), this is reflected in heritable variation in fitness among individuals. In the colonial volvocine algae, the way in which heritable variation in fitness is partitioned within a particular population depends on the developmental program, the mutation rate, and the demography of the population.

At one extreme, we can imagine a pond in which a population is founded by a single colony, which begins reproducing asexually. This is biologically plausible, for example if a pond is colonized through long-distance dispersal. Heritable variation will arise through new mutations and will reside primarily among colonies. Any genetic variation among cells will be fleeting: a mutation arising during development will give rise to a chimeric colony, but each of that colony’s offspring will be genetically homogeneous (some with and some without the mutation), since each daughter colony derives from a single cell.

At the opposite extreme, we can imagine a pond in which the founding population is large and genetically diverse. This situation too is biologically plausible, for example when a summer bloom is initiated by descendants of sexually produced spores from the previous year’s population. In this case, heritable variation in fitness will be found mainly among the clonal lineages descending from different spores. Depending on the mutation rate, additional genetic variation will eventually arise within these lineages due to new mutations.

Considering these two scenarios as extremes along a continuum, we see that the individuals-as-units-of-evolution view implies that the degree of individuality of a given unit is not entirely an inherent property of the unit itself. Rather, the degree of individuality is contingent on the particular ecological and demographic circumstances. Furthermore, as these circumstances change over ecological time, the proportion of heritable variation in fitness found at each level changes as well. This is not only an epistemological distinction. Heritability itself, and not just measurements of heritability, really does change from one generation to the next as allele frequencies change (Lynch and Walsh 1998). As it does, the partitioning of heritable variation in fitness among cells, colonies, and clones will change as well. Contrary to our intuitions, then, the units-of-evolution view suggests that the degree of individuality can change not only through long-term evolutionary change but also through short-term changes in population structure.

I think this is an under-appreciated complication inherent to views of individuality that center around heritable variation in fitness. If that is our criterion for individuality, or for estimating degrees of individuality, we need to consider that the amount and even existence of heritable variation in fitness depend on demographic and ecological circumstances. That is, a population (of organelles, cells, multicellular organisms, or whatever) may have heritable variation in fitness in some conditions and not in others. This suggests that individuality is not just a function of the properties of the individuals, but depends on their external circumstances. It doesn’t mean that such views are wrong, but it does mean that they don’t conform to our intuition (my intuition, at any rate) that individuality, or at least degree of individuality, is something inherent to the units we’re talking about. It doesn’t feel right to say that a Volvox spheroid is an individual under some conditions but not others. That, however, is exactly what views based on heritable variation in fitness imply.

 

Herron, M. D. 2017. Cells , colonies, and clones : individuality in the volvocine algae. In S. Lidgard & L. K. Nyhart, eds., Biological Individuality: Integrating Scientific, Philosophical, and Historical Perspectives (pp. 63–83). University of Chicago Press, Chicago.

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