Histones package DNA and regulate epigenetic states. The most important histone in the latter regard is H3. Mammals have three near-identical H3 isoforms: canonical H3.1 and H3.2, and the replication-independent (RI) variant H3.3. H3.3 can accumulate in slowly dividing somatic cells, where it replaces canonical H3. Some RI histones, through their ability to incorporate outside of S-phase, are functionally important in the very slowly dividing mammalian germ line. However, functions for H3.3 in germ cell development are largely undetermined.We present a comprehensive analysis of the developmental effects of null mutations in each of the two genes encoding H3.3.
H3f3a mutants were viable to adulthood, with males being subfertile with dysmorphic spermatozoa. H3f3b mutants (made by Cre-mediated mutagenesis in the zygote) were growth-deficient, dying at birth. H3f3b heterozygotes were also growth-deficient, with males being sterile because of arrest of round spermatids. This sterility was not accompanied by abnormalities in sex chromosome inactivation in meiosis I: we examined this process as it has been suggested that H3.3 could play a functional role. Conditional ablation of H3f3b at the beginning of folliculogenesis resulted in zygote cleavage failure, establishing H3f3b as a maternal-effect gene, and revealing a requirement for H3.3 in the first mitosis. Simultaneous ablation of both genes in folliculogenesis resulted in the death of early primary oocytes.
These findings reveal a heavy reliance on H3.3 for growth, gametogenesis, and fertilization, identifying developmental processes that are particularly susceptible to H3.3 deficiency. They also reveal partial redundancy in function of H3f3a and H3f3b, while demonstrating the latter to be generally the most important.