Cryptic species of Asplenium nidus complex from the East Malesia region and their speciation process.

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Asplenium nidus is one of the most common epiphytic fern species in the Old World tropics. It has simple leaves without any particular appendages, and the lack of good species recognition might be due to the lack of good taxonomic characters. Recently, we made a detailed analysis of the genetic variation of A.nidus in the East Malesia region (Malay peninsula, Sumatra, Java, Borneo) and Madagascar. We collected leaf materials from many localities and determined rbcL sequences and also made artificial crossing experiments. We found about 50 cryptic species in these regions. Using the results of crossing experiments and rbcL sequence differences between crossed samples, we also discussed evolutionary process of reproductive isolation in this fern group. As the sequence difference got larger, number of hybrids obtained through crossing experiments became smaller. However, in a few cases, reproductive isolation was strong even though rbcL difference was small. It might means reproductive barrier is sometimes formed quickly. It was also observed that hybrids were obtained only when one of the crossed parental race was mother. It means that reproductive isolation is formed only in one direction of crossing. Similar phenomena have been observed in Drosophila and this was considered due to that responsible genes are coded on X-chromosomes. However, ferns including A.nidus are hermaphrodites and they cannot have X-chromosomes. Therefore, it is very interesting that such the one directional crossability was observed in ferns.