Inbreeding depression and outbreeding depression are evident in wild-type zebrafish lines.

Maintaining wild-type (WT) zebrafish stocks for research while preserving viability within the lines used presents significant challenges to zebrafish husbandry practices. Genetic homogeneity is established through inbreeding to provide continuity across experiments. This, however, leads to decreased fitness through inbreeding depression. In the laboratory setting, it is imperative that researchers consistently obtain a large number of viable embryos; thus, inbreeding depression must be suppressed. Genetic variation can be established by creating hybrid lines; however, crosses between genetically distinct lines can cause an outbreeding depression as well. There is little data describing the effects of inbreeding depression or outbreeding depression from such crosses in zebrafish. Additionally, there is a need to establish breeding standards within the zebrafish field. This study examines the susceptibility to inbreeding and outbreeding depression in crosses between four WT zebrafish lines: the inbred lines AB and Tab 14, and the F1 generation of hybrid lines TuAB and TLAB. We report that mating frequency and clutch size were significantly greater in hybrid female crosses than in inbred female crosses. Inbreeding depression in common zebrafish lines such as AB and Tab 14 used here results in fewer successful matings and smaller clutch sizes. Further, outbreeding depression caused by crossing distantly related lines, such as the inbred Tab 14 and the hybrid TLAB lines, can also influence successful zebrafish mating. These data provide evidence needed to further characterize commonly used WT zebrafish lines. We suggest that to maintain lines that mate frequently and yield large clutches, hybrid females of known backgrounds should be used.