Coping with potential bi‐parental inbreeding: limited pollen and seed dispersal and large genets in the dioecious marine angiosperm Thalassia testudinum

Abstract

The high prevalence of dioecy in marine angiosperms or seagrasses (>50% of all species) is thought to enforce cross‐fertilization. However, seagrasses are clonal plants, and they may still be subject to sibling‐mating or bi‐parental inbreeding if the genetic neighborhood is smaller than the size of the genets. We tested this by determining the genetic neighborhoods of the dioecious seagrass Thalassia testudinum at two sites (Back‐Reef and Mid‐Lagoon) in Puerto Morelos Reef Lagoon, Mexico, by measuring dispersal of pollen and seeds in situ, and by fine‐scale spatial autocorrelation analysis with eight polymorphic microsatellite DNA markers. Prevalence of inbreeding was verified by estimating pairwise kinship coefficients; and by analysing the genotypes of seedlings grown from seeds in mesocosms. Average dispersal of pollen was 0.3–1.6 m (max. 4.8 m) and of seeds was 0.3–0.4 m (max. 1.8 m), resulting in a neighborhood area of 7.4 m2 (range 3.4–11.4 m2) at Back‐Reef and 1.9 (range 1.87–1.92 m2) at Mid‐Lagoon. Neighborhood area (Na) derived from spatial autocorrelation was 0.1–20.5 m2 at Back‐Reef and 0.1–16.9 m2 at Mid‐Lagoon. Maximal extensions of the genets, in 19 × 30 m plots, were 19.2 m (median 7.5 m) and 10.8 m (median 4.8 m) at Back‐Reef and Mid‐Lagoon. There was no indication of deficit or excess of heterozygotes nor were coefficients of inbreeding (F IS) significant. The seedlings did not show statistically significant deficit of heterozygotes (except for 1 locus at Back‐Reef). Contrary to our expectations, we did not find evidence of bi‐parental inbreeding in this dioecious seagrass with large genets but small genetic neighborhoods. Proposed mechanisms to avoid bi‐parental inbreeding are possible selection against homozygotes during fecundation or ovule development. Additionally, the genets grew highly dispersed (aggregation index Ac was 0.09 and 0.10 for Back‐Reef and Mid‐Lagoon, respectively); such highly dispersed guerrilla‐like clonal growth form likely increases the probability of crossing between different potentially unrelated genets.

Population genetic structure and connectivity of the seagrass Thalassia hemprichii in the Western Indian Ocean is influenced by predominant ocean currents

“…ual reproduction modes are an important in T. hemprichii (mean R = 0.65). In this study, inbreeding and/or lowly polymorphic microsatellites seem the most likely explanation for the high and significant F IS values, as a study of the closely related Thalassia testudinum at two sites in Mexico found no prevalence of inbreeding(Van Tussenbroek et al, 2016). Significantly heterozygote deficits (positive F IS -values) were observed for all sampled populations, a finding commonly observed in seagrasses in general (Arnaud-Haond, Stoeckel, & Bailleul, 2019) and found in T. hemprichii in the Indo-Australian Archipelago as well (Hernawan et al, 2016).…”

Fine‐scale genetic structure and flowering output of the seagrass Enhalus acoroides undergoing disturbance

“…For example, most of the Z. marina seeds disperse <5 m (Orth, Luckenbach, & Moore, ). The average dispersal distance of pollen and seeds for Thalassia testudinum were also short with about 0.3–1.6 and 0.3–0.4 m, respectively (Van Tussenbroek et al, ), although the fruits can float for kilometers. In our study, significant SGS occurred in 0–2 m for E. acoroides at the genet‐level.…”

“…The Limited dispersal of seeds/pollen is the key factor causing the significant fine-scale SGS of seagrasses at the genet-level (Hämmerli & Reusch, 2003a;Van Tussenbroek, Montero, Wong, Santos, & Guzman, 2010;Van Tussenbroek et al, 2016). For example, most of the Z. marina seeds disperse <5 m (Orth, Luckenbach, & Moore, 1994).…”

Range‐wide population genetic structure of the Caribbean marine angiosperm Thalassia testudinum

“…Fast‐flowing currents are a key environmental factor for T. testudinum's sexual dispersal as the fruits are buoyant. These fruits eventually open and the seeds, which are not buoyant, drop to the sea floor (Van Tussenbroek et al., ). The importance of asexual propagules is unknown and needs to be investigated.…”