Knowledge of eukaryotic life cycles and associated genome dynamics stems largely from research on animals, plants, and a small number of “model” (i.e., easily cultivable) lineages. This skewed sampling results in an underappreciation of the variability among the many microeukaryotic lineages, which represent the bulk of eukaryotic biodiversity. The range of complex nuclear transformations that exists within lineages of microbial eukaryotes challenges the textbook understanding of genome and nuclear cycles. Here, we look in‐depth at Foraminifera, an ancient (∼600 million‐year‐old) lineage (...) widely studied as proxies in paleoceanography and environmental biomonitoring. We demonstrate that Foraminifera challenge the “rules” of life cycles developed largely from studies of plants and animals. To this end, we synthesize data on foraminiferal life cycles, focusing on extensive endoreplication within individuals (i.e., single cells), the unusual nuclear process calledZerfall, and the separation of germline and somatic function into distinct nuclei (i.e., heterokaryosis). These processes highlight complexities within lineages and expand our understanding of the dynamics of eukaryotic genomes. (shrink)