Multiple genetic contributions to plant performance in Thymus vulgaris

1. Diverse genetic factors can affect plant performance. Here we evaluate how maternal and paternal origin, maternal and offspring sexual phenotype and degree of outcrossing jointly influence performance of gynodioecious Thymus vulgaris. 2. In uniform conditions we quantified size and survival of second-generation (G2) off-spring produced by selfing and outcrossing on the open-pollinated offspring (G1) of G0 females from four populations that differ in sex ratio, selfing rate and chemical type. We controlled for environmental variation, maternal effects and plant age. 3. Offspring with maternal parents from populations dominated by plants that synthesize phenolic monoterpenes had greater survival and size than those from populations with non-phenolic chemotypes. In addition, outcrossing among populations with pollen donors from non-phenolic populations significantly reduced progeny performance. These results suggest chemotype-mediated local adaptation. 4. The progeny of hermaphrodites showed significant inbreeding depression, which despite the pollination treatment x family and pollination treatment x population interactions, showed no consistent variation in relation to population inbreeding levels. 5. For a given level of outcrossing, the progeny of hermaphrodites showed greater among-family and among-population variation in plant size than those of females. Variation in nuclear restorer gene composition across cytoplasmic backgrounds may cause this variation. 6. In T. vulgaris, diverse genetic factors jointly affect performance, a result with general implications for understanding fitness variation in natural populations.