Hybrid sterility between two isozymic types of the fernCeratopteris thalictroides in Japan
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JapaneseCeratopteris thalictroides are composed of two allopatric entities called the south type and the north type which are isozymically distinct. Both types are tetraploid at 2n=156. To examine the crossability of these isozymic types, artificial hybridization was conducted among ten representative populations. Allozymes of PGI were used as markers to identify the hybrids among populations. Hybrids within the north type produced normal-shaped spores whose germination rate reached 60.7 to 96.0%. In contrast, hybrids between the north type and the south type formed irregular-shaped spores which were aborted with a low germination rate of 0.0 to 7.0%. It is thus evident that hybrid sterility is present between these types. Morphological analyses revealed that the two types were part of a continuous cline of variability and hence morphologically inseparable. The two types can thus be regarded so far as cryptic species that are morphologically indistinguishable but reproductively isolated.
Key wordsAllozyme Ceratopteris thalictroides Cryptic species Fern Hybrid sterility Reproductive isolation
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