The fern genus Asplenium is well known for its hybridization capacity, especially in temperate zones.
The Appalachian hybrid complex in Asplenium is a polyploid complex composed of three diploid species, three allotetraploid species formed by hybridisation and chromosome doubling from the former, and assorted diploid, triploid and tetraploid hybrids between the six species.
The three diploid parent species are mountain spleenwort (Asplenium montanum), ebony spleenwort (Asplenium platyneuron), and American walking fern (Asplenium rhizophyllum). Their chromosome complements are abbreviated "MM", "PP" and "RR", respectively. Three possible sterile diploid hybrids can form from their hybridization:
In addition, A. platyneuron and A. rhizophyllum, which have a more extensive range, hybridize with other spleenwort species outside the Appalachian complex.
At some point in the past, each of the sterile diploid hybrids experienced a chromosome doubling event that made them fertile allotetraploids; allozyme analysis shows that this happened on several independent occasions for A. montanum × platyneuron and A. montanum × rhizophyllum. These doubling events gave rise to the following tetraploids:
| Common name | Scientific name | Parents | Chromosomes | Notes/Distribution | |
|---|---|---|---|---|---|
| Bradley's spleenwort | Asplenium bradleyi | A. montanum × platyneuron | MMPP | The Appalachians, Ozarks, and Ouachita Mountains. | |
| Lobed spleenwort | Asplenium pinnatifidum | A. montanum × rhizophyllum | MMRR | The Appalachians and the Shawnee Hills. | |
| Tutwiler's spleenwort | Asplenium tutwilerae | A. platyneuron × rhizophyllum | PPRR | Havana Glen, Alabama |
Since these allotetraploids are fertile, they are capable of crossing with each other and with the three diploid species to form the following sterile triploid and tetraploid hybrids, most of which are quite rare and occur sporadically where their parents' ranges overlap:
| Common name | Scientific name | Parents | Chromosomes | Ploidy | Notes/Distribution | |
|---|---|---|---|---|---|---|
| Wherry's spleenwort | Asplenium × wherryi | A. bradleyi × montanum | MMP | triploid | ||
| A. bradleyi × platyneuron | MPP | triploid | Believed to have been collected once at an early date in Pennsylvania; the site is now destroyed. Also reported from Sequatchie County, Tennessee. | |||
| Graves' spleenwort | Asplenium × gravesii | A. bradleyi × pinnatifidum | MMPR | tetraploid | ||
| Trudell's spleenwort | Asplenium × trudellii | A. pinnatifidum × montanum | MMR | triploid | The Appalachians and the Shawnee Hills. | |
| Kentucky spleenwort | Asplenium × kentuckiense | A. pinnatifidum × platyneuron | MPR | triploid | ||
| A. tutwilerae × pinnatifidum | MPRR | tetraploid | Created in culture. | |||
| Boydston's spleenwort | Asplenium × boydstoniae | A. tutwilerae × platyneuron | PPR | triploid | Havana Glen, Alabama | |
| A. tutwilerae × rhizophyllum | PRR | triploid | Created accidentally in culture. |
19 hybrids within the genus have been described from New Zealand, but have not been given binomials.