Reticulate evolution in the Crepidomanes minutum species complex (Hymenophyllaceae).
OBJECTIVE
Hybridization is an important mechanism of speciation in plants and often results in complexes that comprise multiple sexual diploids and their hybrid offspring. However, the intricacy of these systems has prevented a thorough understanding of many groups. The Crepidomanes minutum species complex (Hymenophyllaceae) is a widely distributed, morphologically variable fern species complex. Although prior reports of apogamy and polyploidy suggest hybridization, it has never been the focus of a phylogenetic study.


METHODS
Morphology, nuclear (gapCp), and chloroplast (rbcL) DNA sequences, cytology, field observation, and spore counts were used to infer phylogeny and trace hybrid origins.


RESULTS
The C. minutum species complex is composed of at least three major clades: the African clade, clade 1 (East Asia and the Pacific), and clade 2 (Southeast Asia and the South Pacific). Clades 1 and 2 differ strikingly in morphological variation (uniform in clade 1 vs. highly variable in clade 2) and occurrence of hybrids (rare in clade 1 vs. frequent in clade 2). Apogamy and polyploidy were confirmed as likely mechanisms of hybrid stabilization in clade 2. Despite the large genetic distance between clades 1 and 2, several specimens were observed with gapCp sequences from both; diploid genome size and sexual reproduction indicate maintenance of genetic diversity via introgression or incomplete lineage sorting, rather than ongoing hybridization, in these specimens.


CONCLUSIONS
The C. minutum species complex is a reticulate network including multiple diploid lineages and their stabilized hybrid crosses. Additional sampling focused on reproductive mode and ploidy level is needed to delimit diploid species and hybrids.
Department of System Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan. jnitta@oeb.harvard.edu
Joel H
JH
Nitta
Atsushi
A
Ebihara
Motomi
M
Ito
D000349:Africa; D001208:Asia; D005075:Biological Evolution; D002678:Chimera / Q000235:genetics*; D019143:Evolution, Molecular; D029624:Ferns / Q000033:anatomy & histology / Q000145:classification* / Q000235:genetics*; D014644:Genetic Variation; D005843:Geography; D006824:Hybridization, Genetic; D008969:Molecular Sequence Data; D010139:Pacific Islands; D010802:Phylogeny; D018515:Plant Leaves / Q000033:anatomy & histology*; D011123:Polyploidy; D017422:Sequence Analysis, DNA