B cells participate in constitutive immunological processes to maintain host health and homeostasis. B cells in the mucosa produce a homeostatic barrier of local antibodies, while others take on innate-like phenotypes and patrol bodily cavities, producing homeostatic IgM. There are several unknown aspects of homeostatic B cell function and ontogeny. I primarily employed genetic approaches to address questions about the function of mucosal antibody and of the cellular origins of mucosal antibodies. I developed new mouse models of mucosal antibody deficiency, incl models in I developed robust Cas9-based gene editing methods for the targeted introduction of rearranged immunoglobuins to their corresponding loci and to disrupt the immunoglobulin A locus. My system, called “Speed-Ig,” is a rapid Cas9-based method for generating Ig knock-in mouse lines with high on-target integration rates at both heavy and light chain alleles. With standardized target sites and promoter regions, Speed-Ig mice can be used for comparative studies of B cell biology and vaccine optimization in vivo. I used Speed-Ig to create panels of mice with Ig pairs derived from IgA+ plasma cells, control follicular B-2 cells, or from the innate-like cell lineages B-1a and B-1b understand the cellular ontogeny of mucosal antibody producing cells and innate-like subsets. IgA+ plasma cells appeared to derive from normal follicular B-2 precursors. Surprisingly, B-1b and B-2 Ig pairs drove both B-1b and B-2 phenotypes, suggesting a previously unknown lineage relationship between these subsets. I then confirmed the B-1b/B-2 relationship with transcription factor reporter lines and through adoptive cell transfer experiments. In summary, my Ig knock-in approach facilitated the discovery of previously unappreciated aspect of innate-like B cell biology and revealed a novel B-1b precursor stage present in the peritoneal cavity.