Diatoms favor their younger daughters

We used a time-lapse imaging approach to examine cell division in the marine centric diatom Ditylum brightwellii and observed that daughter cells who inherited their parents’ hypothecal frustule half were more likely to divide before their sisters. This is consistent with observations in Escherichia coli of a bias between sister cells, where faster growth in one sister is thought to arise from its inheriting parental material with less oxidative damage. We also observed that hypothecal sisters in D. brightwellii were more likely to inherit a greater proportion of their parents’ cellular material, similar to what has been seen in E. coli. We found a statistically significant correlation between the amount of parental material inherited by a hypothecal daughter and its relative division rate, indicating that this extra material inherited by the hypothecal daughter plays a role in its more rapid division. Furthermore, the intercept in this regression was greater than zero, indicating that other factors, such as differences in the quality of inherited material, also play a role. This similarity between two taxonomically distant microbes suggests that favoritism toward one daughter might occur broadly among unicellular organisms that reproduce asexually by binary fission. Such a bias in cell division might be advantageous, given model predictions that show that favoring one daughter at the expense of the other can result in higher population growth rates, increasing the chance that a cell’s genotype will survive compared to a model where the daughters divide at equal rates. Many unicellular organisms reproduce primarily by binary fission, an asexual process in which a parent cell undergoes first mitosis, then cytokinesis, to form two new daughter cells. Mitosis ensures that these sister cells are effectively identical in terms of genotype, but it is likely that they will differ at least slightly in phenotype. Phenotypic differences between two nascent sister cells can arise from stochastic effects during cell division that result in the inexact division of parental material between the two progeny. Non-stochastic mechanisms can also bias the allocation of parental material between two sister cells, sometimes systematically and predictably. The degree to which phenotypic differences occur between sister cells— and whether these differences reflect purely stochastic or instead biased processes—may have important consequences not only to the relative growth and fitness of these two cells as individuals, but also at the population level, depending on how these between-sister asymmetries are manifested. A well-known example of a systematic, between-sister phenotypic asymmetry is seen in the diatoms, arising from the morphology of their rigid exterior frustules. Diatom frustules are composed of two halves of slightly different diameters, and during cell division one daughter inherits the parent’s larger-diameter epithecal half and its sister inherits the smaller-diameter hypothecal half. Each daughter then forms a new hypotheca to fit inside whichever frustule half it inherited (Macdonald 1869; Pfitzer 1869). A consequence of this between-sister morphological asymmetry is that the sister inheriting the epitheca will be the same diameter as its parent and the other will be slightly smaller. Although this difference in diameter is slight, over many generations it will lead to the well-known diminution in cell