Severe inbreeding depression in an ecologically important grass is revealed by examining germination, not seed production

TL;DR

Wiregrass produces many seeds through self-pollination, but these seeds rarely germinate due to severe inbreeding depression, highlighting the need to test seed viability in restoration efforts.

Contribution

The study reveals that germination data is essential to detect inbreeding depression, which is masked by seed production alone in grasses.

Findings

Wiregrass is not pollen-limited but exhibits severe inbreeding depression in selfed seeds.

Germination data revealed near-zero viability of selfed seeds, indicating an obligate outcrossing mating system.

Frequent fires likely promote pollen movement and successful outcrossing in wiregrass.

Abstract

Pollen availability and self-compatibility studies in grasses inform restoration by identifying limitations to germinable seed production. Seed set alone indicates pollen limitation and self-fertilization ability but not self-compatibility, as seeds may not germinate. We tested whether wiregrass (Aristida beyrichiana) is pollen-limited and self-compatible in two southeastern US pine savannas differing in ecology and fire frequency. Pollen limitation and self-compatibility were assessed by comparing seed set and germination from naturally pollinated inflorescences, those supplemented with outcross pollen, and those restricted to self-pollen. All treatments produced seeds, but filled and germinable seed numbers varied by site. The wetter, triennially burned site produced significantly more germinable seeds than the drier, annually burned site. Pollen was not limiting: open and outcross-supplemented inflorescences produced similar seed numbers within each site. Based on seed production, wiregrass appeared capable of self-fertilization, but germination data revealed near-zero viability of selfed seeds, indicating severe inbreeding depression rather than true self-incompatibility. Thus, wiregrass is not pollen-limited but exhibits strong embryonic inbreeding depression, implying an obligate outcrossing mating system. This helps explain why wiregrass often produces few viable seeds despite high seed production. Our findings highlight the importance of including germination data in reproductive studies, as seed production alone can mask other biological constraints. For wiregrass, frequent fires likely promote pollen movement and successful outcrossing. These insights inform restoration efforts by clarifying reproductive limitations in a foundational pine savanna species. This has important implications for restoration efforts, particularly in sourcing and evaluating seed material for ecological resilience and long-term establishment.

Wiregrass produces abundant seeds from self-pollination, but germination tests revealed these seeds almost never sprout. This severe inbreeding depression was invisible when counting seeds alone, demonstrating a critical flaw in how plant reproduction is assessed. For effective restoration and accurate pollination studies, seed quality testing is essential. Our findings show that what appears viable may not be, with important implications for understanding plant mating systems and improving restoration outcomes.

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