Research in this proposal is designed to develop and investigate a novel physiologic model for selection of a single dominant follicle. The physiology underlying differences in follicle size at follicle diameter deviation, the morphological manifestation of follicle selection, will be examined. One key hypothesis is that estradiol not only inhibits constitutive FSH secretion but also stimulates amplitude of FSH pulses, potentially explaining paradoxes in current functional-coupling models of FSH and follicle selection. Proposed experiments will test, amend, and validate our working model.The overall goal of the two experiments in Objective 1 is to characterize precise hormonal dynamics associated with follicle deviation at different sizes in heifers and lactating cows. In lactating cows, we will also characterize how hormonal dynamics change in association with selection of two dominant follicle (co-dominant follicles).For Objective 2, two manipulative experiments are proposed to use data on hormonal dynamics to first examine whether physiological concentrations of circulating steroids explain differences in LH and FSH pulse patterns and, second, directly test if differences in pulse patterns explain differences in follicle dynamics.For Objective 3, two experiments will be done with the overall goal of understanding the changes in gene expression that accompany selection of a dominant follicle. These experiments will use RNA-seq methodology to determine transcriptomic changes during earliest stages of follicle selection using two tightly-synchronized models of follicle selection, one in response to increased FSH after dominant follicle ablation near deviation and the other after acute induction of LH action during GnRH-antagonist treatment.For Objective 4, one experiment will be done to test whether double ovulation in dairy cattle is due to GnRH pulses. This experiment has the goal of testing the practical implications of this new physiologic model in explaining an important management problem, increased twinning in dairy cattle.Thus, completion of this research will allow deeper understanding of follicle selection allowing rational design of novel reproductive management strategies and reproductive biotechnologies.