Germination syndrome divergence among pairs of sympatric sister species along an estuarine salinity gradient

Authors: CASTILLO, JESUS - University Of Seville; CURADO, GUILLERMO - University Of Seville; MUNOZ-RODRIGUEZ, ADOLFO - Universidad De Huelva; Grewell, Brenda

Submitted to: Environmental and Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2020
Publication Date: 9/24/2020
Citation: Castillo, J.M., Curado, G., Munoz-Rodriguez, A.F., Grewell, B.J. 2020. Germination syndrome divergence among pairs of sympatric sister species along an estuarine salinity gradient. Environmental and Experimental Botany. 181. Article 104274. https://doi.org/10.1016/j.envexpbot.2020.104274.
DOI: https://doi.org/10.1016/j.envexpbot.2020.104274

Interpretive Summary: A central aim of ecology is to understand the distribution and abundance of co-occurring species, and factors underlying these organized community patterns. Endemic plant species that have restricted geographic ranges tend to be scarce whereas widespread native or invasive species often occur at high densities. There is a vital need to understand the processes and mechanisms that link the abundance and coexistence of plant species and their distribution to improve restoration and management. Depending on these mechanisms, protection or restoration of small patches of high-quality habitat may not achieve conservation goals for endemic species if restricted ranges lead to low population density, whereas successful control of invasive weed species at high density may require local eradication to prevent range expansion. There is a need to evaluate the role of fine-scale factors, such as local abiotic stress gradients, to clarify habitat preferences, phenotypic and ecological differentiation among closely related and co-occurring plant species. Niche segregation is a process by which competing species use the microhabitats within an environment differently in a way that supports their abundance and coexistence. Closely related plant species co-occur within heterogeneous habitats by occupying different ecological niches as the result of an evolutionary process known as divergent adaptive selection. Soil salinity is one of the most important factors influencing vegetation distribution in salt marshes, where germination is a critical stage in the life cycle of many halophytes. Sea lavenders (Plumbaginaceae; leadwort plant family) are a species-rich group of halophytes in coastal plant communities. The center of their diversity is the Mediterranean region where 70% of species are endemic. Three species are native to North America, while at least six others are horticultural escapees that have naturalized and spread as highly invasive weeds. We characterized phenotypic divergence among two pairs of closely related, sympatric sister species of sea lavenders, by evaluating their germination traits and distribution patterns across a wide salinity range in both controlled experimental conditions, and in salt marsh study sites. The four species segregate into two germination syndromes, each with a polyploid Limonium species and diploid Myriolimon species pair. L. narbonense and M. ferulaceum have broad geographical distributions, and their abundance within salt marsh study sites was independent of soil salinity levels. Their germination syndrome was characterized by high germination rates (c. 95%) in freshwater, constant and high germination speed independent of salinity concentration, and reduced total germination (lower than control) after high salinity exposure. In contrast, the syndrome of L. algarvense and M. diffusum, two regionally endemic species, features moderate germination levels (c. 30-80%) over a wide salinity range (0-200 mM) with a reduction of germination speed at higher salinities. Furthermore, their capacity for recovery germination after high salinity exposure supports their abundance in salt marshes. These results suggest intertidal salinity influences on variation in germination traits among pairs of sympatric sea lavenders underlies their abundance, niche segregation, and phenotypic divergence in salt marshes.

Technical Abstract: Closely related plant species co-occur within heterogeneous habitats by occupying different ecological niches as the result of divergent selection. Soil salinity is one of the most important factors influencing vegetation distribution in salt marshes, where germination is a critical stage in the life cycle of many halophytes. We characterized phenotypic divergence among two pairs of sympatric sister species of sea lavenders (Plumbaginaceae), by evaluating their germination traits and distribution patterns across a wide salinity range in both controlled experimental conditions, and in salt marsh study sites. The four species segregate into two germination syndromes, each with a polyploid Limonium species and diploid Myriolimon species pair. L. narbonense and M. ferulaceum have broad geographical distributions, and their abundance within salt marshes was independent of soil salinity levels. Their germination syndrome was characterized by high germination rates (c. 95%) in freshwater, constant and high germination speed independent of salinity concentration, and reduced total germination (lower than control) after high salinity exposure. In contrast, the syndrome of L. algarvense and M. diffusum, two regionally endemic species, features moderate germination levels (c. 30-80%) over a wide salinity range (0-200 mM) with a reduction of germination speed at higher salinities. Furthermore, their capacity for recovery germination after high salinity exposure supports their abundance in salt marshes. These results suggest intertidal salinity influences on variation in germination traits among pairs of sympatric sea lavenders underlies their abundance, niche segregation, and phenotypic divergence in salt marshes.