Abstract
Hybridization is a ubiquitous force in plant evolution. In ferns, hybrids are often easily recognized by their intermediate morphology and abortive spores and thus provide a good model for studying reproductive isolating barriers between species. Asplenium ×aran-tohanum is a rare fern hybrid, despite wide coexistence of its parental species (A. billotii and A. trichomanes subsp. quadrivalens) in Western Europe. We made a complete characterization of its three known individuals, including macro- and micromorphology, sporogenesis, gametophyte reproduction, and chloroplast DNA inheritance and evolution. The hybrid expressed morphological characters that were mostly intermediate between those of the parents, but some characters were more similar to one parent or the other. Sizes of both guard cells and spores indicate that the hybrid is tetraploid, as are both parents, and one parent (A. billotii) consistently acted as the female. A very small fraction of spores (~ 7%) were viable and the resulting gametophytes could not form sporophytes, either sexually or apogamously, suggesting that effective postzygotic barriers exist between the parents. The lineages of these taxa diverged about 35 million years ago, which may explain the strong reproductive isolation and rarity of the hybrid. Asplenium ×aran-tohanum appears to be an evolutionary dead end, probably formed recurrently at the places where it grows, but incapable of completing its life cycle or producing viable offspring.
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Acknowledgements
We are grateful to Juan A. Alejandre for sharing material from the type plant and to Miguel Pachón, who came to the field trip around Madrid and discovered the hybrid plant in this locality. The Universidad Complutense de Madrid partially funded this work: a) Research Groups Funding Programs, “910801: Taxonomic Botany and Diversity”; and b) Research Project PR26/16-20295. We thank two anonymous reviewers for their useful suggestions.
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Online Resource 1. Alignment used to produce the phylogeny.
Online Resource 2. Critical gametophytic events and structures of Asplenium ×aran-tohanum.
Appendix 1. List of material
Due to the existence of just three sampling spots, the material is ordered by location at the first level (alphabetical order of Spanish Province), with indication of geographical coordinates and altitude over the sea level. For each sample, basic collection data (collector, date and herbarium voucher) is given. ENA (European Nucleotide Archive) accesions are included for the markers trnG–trnR, rbc–L, and rps4–trnS, in this order.
ÁVILA: Solosancho, 40.52 -4.90, 1380 m a. s. l. Asplenium ×aran-tohanum Alejandre & M.J.Escal.: Alejandre and Escalante, 18 Dec 2004, Herbarium Alejandre 1407/04; LR588501, LR588504, LR588496. Asplenium billotii F.Schultz: Gabriel y Galán, 26 Feb 2018, MACB 112993; LR585233, LR585211, LR586053. Asplenium trichomanessubsp. quadrivalens D.E.Mey: Gabriel y Galán, 26 Feb 2018, MACB 112994; LR585230, LR585209, LR586055.
A CORUÑA: Santiago, 42.88 8.55, 230 m a. s. l. Asplenium ×aran-tohanum Alejandre & M.J.Escal.: L.G. Quintanilla 641 and B. Pías, 5 Nov 1998, MACB 112961; LR588499, LR588502, LR588498. Asplenium billotii F.Schultz: L.G. Quintanilla 643 and B. Pías, 5 Nov 1998, MACB 112959; LR585232, LR585212, LR586054. Asplenium trichomanessubsp. quadrivalens D.E.Mey: L.G. Quintanilla 642 and B. Pías, 5 Nov 1998, MACB 112960; LR585228, LR585210, LR586056.
MADRID: La Cabrera, 40.84 -3.67, 1080 m a. s. l. Asplenium ×aran-tohanum Alejandre & M.J.Escal.: Molino et al., 19 May 2016, MACB 112692; LR588500, LR588503, LR588497. Asplenium billotii F.Schultz: Molino et al., 19 May 2016, MACB 112958; LR585231, LR585213, LR586052. Asplenium trichomanessubsp. quadrivalens D.E.Mey: Molino et al., 19 May 2016, MACB 112957; LR585229, LR585208, LR586057.
Appendix 2. Additional sequences
GenBank accessions, ordered by taxa, for the markers trnG–trnR, rbc–L, and rps4–trnS, in this order, corresponding to the taxa used for constructing the phylogenetic tree.
Asplenium adiantum-nigrum L.: KP861371, KP899628, KR233955. Asplenium billotii F.Schultz: KC792645, KC792629, KF923967. Asplenium fontanum (L.) Bernh.: KC792642, KC792625, KC792642. Asplenium hemionitis L.: KU753794, KP899633, JQ364916. Asplenium marinum L.: KP861374, KU753804, AY549773. Asplenium obovatum Viv. subsp.obovatum: KC792639, KC792628, KF923970. Asplenium obovatumsubsp. protobillotii Demiriz, Viane & Reichst.: KC792638, KC792635, KF923973. Asplenium onopteris L.: KC792636, KC792632, KF923965. Asplenium petrarchae (Guérin) DC.: KP861366, KP899640, AY549804. Asplenium ruta-muraria L.: KP861361, KP899645, AY549763. Asplenium scolopendrium L.: KP861370, KP899647, AY459169. Asplenium septentrionale (L.) Hoffm.: KP861377, KP899649, AY549777. Asplenium subglandulosumsubsp. hispanicum (Coss.) Salvo, Prada & T.E.Díaz: KU753793, KP899651, JX068786. Asplenium trichomanessubsp.quadrivalens D.E.Mey.: KP861388, KP899654, AY549794. Asplenium viride Huds.: KP861387, KP899656, EF645627. Hymenasplenium unilaterale (Lam.) Hayata: JF832224, EF452140, AY459170.
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de la Fuente, P., Gabriel y Galán, J.M., Molino, S. et al. Character expression, reproductive barriers, and origin of the rare fern hybrid Asplenium ×aran-tohanum (Aspleniaceae). Plant Syst Evol 306, 24 (2020). https://doi.org/10.1007/s00606-020-01658-8
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DOI: https://doi.org/10.1007/s00606-020-01658-8