Polyploidization is generally considered a major evolutionary force that can alter the genetic diversity, morphology, physiology and ecology of plants. One striking example is the polyploid Urtica dioica complex, in which diploid taxa are often found in remote and partly relictual geographical ranges, in contrast to tetraploid individuals, which have an unknown evolutionary history and occur in a variety of synanthropic habitats. We used a set of 279 plants, evenly representing the geographical and morphological variation of U. dioica s.l. in Europe and Southwest Asia, and employed multivariate and geometric morphometrics and Hyb-Seq sequencing to estimate the extent of differentiation of diploid taxa and the ubiquitous tetraploid cytotype. Diploid subspecies form more-or-less separate clusters in morphological analyses, but our molecular evaluation did not reveal any structure. Moreover, tetraploids coalesced with diploids in both morphological and molecular analyses. This disparity between morphological and molecular data might be driven by (1) local adaptation of the diploid cytotype that is mirrored in specific phenotypes, (2) only recent genetic diversification of the group and (3) homoploid and heteroploid hybridization events.
The performance of first‐generation hybrids determines to a large extent the long‐term outcome of hybridization in natural populations. F1 hybrids can facilitate further gene flow between the two parental species, especially in animal‐pollinated flowering plants. We studied the performance of reciprocal F1 hybrids between Rhinanthus minor and R. major, two hemiparasitic, annual, self‐compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. Germination under laboratory conditions was much lower for the F1 hybrid formed on R. major compared with the reciprocal hybrid formed on R. minor, and this confirmed previous results from similar experiments. However, this difference was not found under field conditions, which seems to indicate that the experimental conditions used for germination in the laboratory are not representative for the germination behaviour of the hybrids under more natural conditions. The earlier interpretation that F1 hybrid seeds formed on R. major face intrinsic genetic incompatibilities therefore appears to be incorrect. Both F1 hybrids performed at least as well as and sometimes better than R. minor, which had a higher fitness than R. major in one of the two years in the greenhouse and in the field transplant experiment. The high fitness of the F1 hybrids confirms findings from naturally mixed populations, where F1 hybrids appear in the first year after the two species meet, which leads to extensive advanced‐hybrid formation and introgression in subsequent generations.
The performance of first-generation hybrids determines to a large extent the long-term outcome of hybridization in natural populations. F1 hybrids can facilitate further gene flow between the two parental species, especially in animal-pollinated flowering plants. We studied the performance of reciprocal F1 hybrids between Rhinanthus minor and R. major, two hemiparasitic, annual, self-compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. While germination under laboratory conditions was much lower for the F1 hybrid formed on R. major compared to the reciprocal hybrid formed on R. minor, this difference disappeared under field conditions, pointing at an artefact caused by the experimental conditions during germination in the lab rather than at an intrinsic genetic incompatibility. Both F1 hybrids performed as well as or sometimes better than R. minor, which had a higher fitness than R. major in one of the two years in the greenhouse and in the field transplant experiment. The results confirm findings from naturally mixed populations, where F1 hybrids appear as soon as the two species meet and which leads to extensive advanced-hybrid formation and introgression in subsequent generations.
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