Parental Sacrifice

A mother’s willingness to sacrifice for the sake of her children is innate in many species besides humans. Female polar bears starve to nurse cubs. Dolphin mothers stop sleeping to care for newborns. Some spider moms make a meal of themselves for their hatchlings.

Yeast asexually propagate by budding; a variant of mitosis. A small daughter cell forms. This bud grows until it separates from mom.

A yeastling is typically smaller than its mother. This contradicts the classic picture of mitosis, which postulates an even splitting of cells into two identical copies.

But uneven division is not uncommon. Human stem cells often divide into cells that look and behave quite differently.

A yeast mother’s size reflects her generational age. Yeast grow as they get older.

Mitochondria are the power plant organelle for all eukaryotic cells. Each cell meets its energy needs by mitochondrial production.

The ratio of a yeast’s mitochondrial size to total cell size is not constant. Mitochondrial ratio declines as a yeast gains girth and ages.

As a bud grows, a yeast mother consistently provides sufficient mitochondria to the bud to ensure her daughter’s survival. In the process, mom gives more than she can recover. Hence, with each generation, a yeast mother sacrifices for her offspring.

Most yeast begin to die off after 10 generations. Few survive to 20.


Ishi Nobu, Spokes of the Wheel, Book 2: The Web of Life (pre-publication)

Susanne M. Rafelski et al, “Mitochondrial network size scaling in budding yeast,” Science 338(6108): 822–824 (9 November 2012).