Hybrid Ferns of the Soller Valley

 

The Soller Valley, located on the northwest coast of Majorca Island ( Spain ), near the sea and surrounded by protective mountains, is famous for its extraordinary geographic beauty, for its excellent oranges, for the high quality of its olive oil, for its port in calm waters, for its welcoming inhabitants as well, although much less known, for its rich flora of pteridophytes, especially small ferns of Aspleniaceae family, characterized by their strong tendency to hybridization between them.

 

________                            Asplenium majoricum

                                               ( sunny habitat )

______                             Asplenium majoricum

                                            ( shaded habitat )

                                       Asplenium X reichsteinii

 

                                 Asplenium X helii nothosubsp. lainzii

_______                             Asplenium X sollerense

_________                           _    Asplenium X orellii

________                      _Asplenoceterach_barrancense

_________                        Ceterach officinarum ssp. officinarum

 



Most hybrid ferns, if not all, have a polyploid chromosomal envelope (triploid, tetraploid, hexaploid, etc. ..). The tetraploids, with double chromosomal strength, are very common among the ferns and may have two origins:

 

-Allotetraploids: by cross between two different diploid species (interspecific hybridization), resulting in a diploid hybrid fern. In most cases these allodiploid hybrids are sterile, produce aborted spores and die without leaving offspring. However, sometimes it resorts to unorthodox strategies, genetically speaking, and get perpetuated. Its secret lies in a mutation of the gene that encodes the process of meiosis, called Apomeiosis (no meiosis). At the time of meiosis to take place within the sporangium, to halve the number of chromosomes to the spores, which are normally haploid ( haplospores ), rather than one mitosis meiosis occurs, giving rise to diploid spores ( diplospores ) that germinate and then produce diploid gametophytes. The arquegonium ( female ) and the antheridium ( male ) of the gametophytes generates diploid oospheres and antherozoids which, once fertilized, give rise to allotetraploid sporophytes, most achievable. They have all the chromosomes of the father and all of the mother, not missing any gene necessary for life. This alteration in meiosis is relatively common in the plant world, especially among the ferns. This has made many new species.

 

 

  Diploid Fern ( PP )                                   Diploid Fern ( FF )

( Asplenium petrarchae ssp. bivalens )                                                    ( Asplenium fontanum )

   ( 2n = 72 chromosomes)                                                                             ( 2n = 72 chromosomes)

            V                                                                                                                 V

                    V                                                                                                                 V

                    V                                                                                                                 V

         meiosis                                                                             meiosis

               V                                                                                                                 V

                    V                                                                                                                 V

                    V                                                                                                                 V

    Haploid Spores ( P )                                                     Haploid Spores ( F )

       ( n = 36 chromosomes )                                                                         ( n = 36 chromosomes )

                    V                                                                                                                 V

                    V                                                                                                                 V

                    V                                                                                                                 V

        germination                                                                    germination

               V                                                                                                                 V

                    V                                                                                                                 V

                    V                                                                                                                 V

Haploid Gametophytes                                                 Haploid Gametophytes

       ( n = 36 chromosomes )                                                                             ( n = 36 chromosomes ) 

               V                                                                                                                  V

                    V                                                                                                                  V

                    V                               INTERSPECIFIC HYBRIDIZATION                        V

  Haploid Gametes ( P )-------------------X -----------------Haploid Gametes ( F )

   ( oospheres and antherozoids )                            V                              ( oospheres and antherozoids )

                                                                                   V

                                                                                   V

                                                                       fecundation

                                                         ( 36 + 36 )

                                                                                   V

                                                                                   V

                                                                                   V

                                                           Allodiploid Sporophyte ( FP )

                                                  ( Asplenium X protomajoricum )

                                                 ( 2n = 72 chromosomes )

                                                                  V                                  V

                                                                  V                                  V

                                                                  V                                  V

                                                       unviable meiosis                   V

                                                                                                       V

                                                                                          apomeiosis

                                                                                 V

                                                                                                       V

                                                                                      Diploid Spores ( FP )

                                                                  ( n = 72 chromosomes )

                                                                                 V

                                                                                                       V

                                                                                                       V

                                                                                          germination

                                                                             V

                                                                                                       V

                                                                                                       V

                                                                                 Diploid Gametophytes

                                                               ( n = 72 chromosomes )

                                                                                                       V

                                                                                                       V

                                                                                                       V

                                                                                   Diploid Gametes (FP )

                                                              ( oospheres and antherozoids )

                                                                                 V

                                                                                                       V

                                                                                                       V

                                                                              endogamic fecundation

                                                                        ( 72 + 72 )

                                                                                 V

                                                                                                       V

                                                                                                       V

                                                                         Allotetraploid Sporophyte ( FFPP )

                                                                               ( Asplenium majoricum )

                                                                                 ( 2n = 144 chromosomes )

 

 

An example of interspecific hybrid allotetraploid fern is Asplenium majoricum which, although produced many aborted spores, also produces many perfectly viable ( diplospores ), so it feels very prolific and has colonized the mountains of Soller, Fornalutx and Deia. It comes from the spontaneous hybridization between two diploid ferns: Asplenium fontanum (FF) and Asplenium petrarchae ssp. bivalens (PP), resulting in a sterile allodiploid hybrid, Asplenium X protomajoricum (FP), unable to reproduce itself by lack of homology between the genomes, but relies on the apomeiosis for the perpetuation, producing a son with chromosomal double envelope, the foregoing Asplenium majoricum (FFPP ).

 

-Autotetraploids: by doubling the chromosomal strength of a diploid fern (no interspecific hybridization). As in the case of allotetraploids, because the same mutation that encodes a meiosis, mitosis occurs where there should be a meiosis (Apomeiosis) in sporangia of a diploid normal fern, so that instead of normal haploid spores are produced diploid spores (diplospores), which germinate to give rise to diploid gametophytes, which in turn generate diploid gametes and, after fertilization, produce a tetraploid sporophyte, autotetraploid in this case,  it simply have double number of chromosomes or two identical genomes, the union of a diploid oosphere and a diploid antherozoide of the same species of fern. In nature there are three well-known examples of autotetraploid ferns perfectly viable and very abundant: Asplenium Trichomanes ssp. quadrivalens, Asplenium petrarchae ssp. petrarchae and Ceterach officinarum ssp. officinarum.

 

Because we already have an allotetraploid fern by hybridization and an autotetraploid fern by genomic duplication. Now comes the hard: either one as the other, to achieve viable offspring, should be played with two whole genomes in the nucleus of cells. To achieve this could follow two paths: the normal sexual reproduction itself of any plant, so they have to produce viable diploid spores with an appropriate number of chromosomes that respects parity, so the child does not need any gene necessary for life, something perfectly possible in autotetraploids but more difficult in allotetraploid interspecific hybrids, which generate many aborted unviable spores and a few viable, enough to perpetuate itself.

 

 

  Asplenium fontanum ( 2n)-----------------------------------------------------------------------------------------------------------> Asplenium X reichsteinii (3n)

             ,    (FF)                                                                                     ,                                                                                     (FFP)

             ,                                                                                               X

             ,                                                                                                ,

            X----------------------------------------------------------------> Asplenium majoricum (4n)

             ,                                                                                                ,  (FFPP)

             ,                                                                                                ,

             ,                                                                                                ,

  Asplenium petrarchae ssp. bivalens (2n)                                          ,

                   (PP)                                                                                    ,

                                                                                                              ,

  Asplenium petrarchae ssp. petrarchae (4n)----------------------> X -------------------------------------------> Asplenium X sollerense (4n)

                   (PPPP)                                                                                ;                                                                                    (PPPF)

                                                                                                              ,

 Ceterach officinarum ssp. officinarum(4n)------------------------> X -------------------------------------------> Asplenoceterach barrancense ( 4n )

                  (CCCC)                                                                               ,                                                                                      (CCFP)

                                                                                                              ,

  Asplenium trichomanes ssp. quadrivalens (4n)-------------------> X -------------------------------------------> Asplenium X orellii (4n)

            ,      (QQQQ)                                                                                                                                                                     (QQFP)

           

           X----------------------------------------------------->Asplenium X helii nothosubsp. lainzii (4n)

            ,                                                                                                (QQCC)

  Asplenium petrarchae ssp. petrarchae(4n)

                  (CCCC)

                                                                                                                                                                                    

                                                                                                                        

  Asplenium azomanes (4n)                                                               

           ,       (AAAA)                                                                                                                                                                    

                                                                                                            

           X-----------------------------------------------------> Asplenium X tubalense (4n)

           ,                                                                                               (QQAA)

 Asplenium trichomanes ssp. quadrivalens (4n)

            ,     (QQQQ)             

          

           X ----------------------------------------------------> Asplenium trichomanes nothosubsp. lucanum (3n)

            ,                                                                                                (QQN)

  Asplenium trichomanes ssp. inexpectans (2n)

                   (NN)

 

 

Another special case are the triploid hybrids, most of which having an odd number of chromosomes are sterile due to the impossibility of forming viable gametes during meiosis. Take for example the case of Asplenium X reichsteinii (FFP), the result of crossbreeding of Asplenium majoricum (FFPP) with his father Asplenium fontanum (FF). Following the normal process of sexual reproduction, the possible combinations of genomic spores would be the most viable (aborted) by a number of aberrant chromosomes, a combination of genes is incompatible with life or even the total absence of chromosomes. There not seems the sexual way used by this small fern to perpetuate itself. So why that Asplenium X reichsteinii is so abundant in some places of the Soller Valley?. A sterile hybrid is usually a solitary individual, anecdotal, it dies without offspring. One possible explanation is that it skips the rules, passing long by the meiosis (apomeiosis), as we have seen before with the tetraploids, producing triploid gametophytes without genomic reduction, and it skips also the fertilization and it uses another asexual reproductive strategy, the Gametophytic Apomixis, a mutation that allows growth of a fern from an adult somatic cell from gametophyte by parthenogenesis. The resulting child has exactly the same genome that his parent, it is an identical clone. With this dual strategy, with two mutations that affect the reproductive process, is perfectly feasible to have children clones, identical copies to fern-mother.

 

 

Diploid Fern                                                       Tetraploid Fern

( 2n = 72 chromosomes)                                                                                      ( 2n = 144 chromosomes )

                V                                                                                                                            V

                V                                                                                                                            V

                V                                                                                                                            V

         meiosis                                                                                   meiosis

           V                                                                                                                             V

               V                                                                                                                             V

               V                                                                                                                             V

 Haploid Spores                                                                       Diploid Spores

( n = 36 chromosomes )                                                                                        ( n = 72 chromosomes )

               V                                                                                                                             V

               V                                                                                                                             V

               V                                                                                                                             V

     germination                                                                           germination

           V                                                                                                                             V

               V                                                                                                                             V

               V                                                                                                                             V

Haploid Gametophytes                                                    Diploid Gametophytes

   ( n = 36 chromosomes )                                                             ( n = 72 chromosomes )

               V                                                                                                                             V

               V                                                                                                                             V

               V                                           INTERSPECIFIC HYBRIDIZATION                      V

 Haploid Gametes -----------------------------X ---------------------Diploid Gametes

  ( oospheres and antherozoids )                                   V                                     ( oospheres and antherozoids )

                                                                                          V

                                                                                          V

                                                                                          V

                                                                             fecundation

                                                              ( 36 * 72 )

                                                                                          V

                                                                                          V

                                                                                          V

                                                                                          V

                                       Triploid Sporophyte

                                         ( Triploid fern with 2n =108 chromosomes )

                                                                      V                                            V

                                                                      V                                            V

                                                                      V                                            V

                                                              unviable meiosis                         V

                                                                                                                    V

                                                                                                           apomeiosis

                                                                                       V

                                                                                                                    V

                                                                                                                    V

                                                                                          Triploid Spores (triplospores)

                                                                        ( 2n = 108 chromosomes )

                                                                                                                    V

                                                                                                                    V

                                                                                                                    V

                                                                                                       germination

                                                                                       V

                                                                                                                    V

                                                                                                                    V

                                                                                               Triploid Gametophytes

                                                                          ( 2n = 108 chromosomes )

                                                                                                                    V

                                                                                                                    V

                                                                                                                    V

                                                                                                  Triploid Gametes

                                                                      ( oospheres and antherozoids )

                                                                                                    V                                 V

                                                                                                    V                                 V

                                                                                                    V                                 V

                                                                                   unviable fecundation                V

                                                                                                                                        V

                                                                                                                    gametophytic apomixis

                                                                             ( growth of a somatic cell of gametophyte )

                                                                                                                                         V

                                                                                                                                         V

                                                                                                                                         V

                                                                                                                    Triploid Sporophyte

                                                                                        ( 2n = 108 chromosomes---> identical fern of its mother = clonic fern )

 

 

Asplenium majoricum, one of the most famous hybrids, was described for the first time in the Soller Valley in 1911 and gave him the name of Majorca, because at first it was believed endemic of the island, but later was also found in the Valencian Community and recently in the south of Catalonia. This tiny fern is very promiscuous and has been retro-hybridized with its parent Asplenium fontanum, giving rise to the hybrid triploid Asplenium X reichsteinii, very vigorous, with pendular fronds and more similar his grandfather-father, with whom he share 66% of the genome; with the natural autotetraploid Asplenium petrarchae ssp. petrarchae, giving rise to the hybrid Asplenium X sollerense, that it seems a giant A. majoricum, with wide and pendular fronds, great and hairy pinnae, with a clear and intense green colour; with the autotetraploid Asplenium Trichomanes ssp. quadrivalens, giving rise to Asplenium X orellii, that at first view seems a vigorous A. majoricum, with the fronds slightly extended, pinnae without trichomes and a showy black or dark garnet color rachis, only green in the end of the frond and, finally, with the autotetraploid Ceterach officinarum ssp. officinarum, giving rise to very small hybrid allotetraploid Asplenoceterach barrancense, with an exactly intermediate phenotype, that it indicates without doubt who are its ancestors.

 

The two progenitors and their hybrid son growing in same calcareous stone.

 

                               Asplenium anceps ( Madeira )

                      Asplenium azoricum ( Faial – Azores )

Asplenium azomanes

( shaded habitat )

Asplenium azomanes

 ( sunny habitat )

______________________Asplenium_X_tubalense

____________Asplenium_trichomanes_nothosubsp._lucanum

 


Other prominent hybrid among Aspleniaceae family that can be found in the Soller Valley is Asplenium azomanes ( for the german botanists it is the Asplenium trichomanes ssp. coriaceifolium ), a fern formed six million years ago during the Messinian period, at the end of the Miocene, by the natural hybridization between the allotetraploid Asplenium azoricum ( descendant of the Asplenium anceps ), which now lives only in the Azores Islands and some specimen of the Asplenium trichomanes complex (perhaps the subspecies hastatum), when the water level of the Mediterranean Sea dropped and it was reduced to a few very salty lakes, due to tectonic movement of European and African plates, which closed the communication between the Atlantic Ocean and Mediterranean Sea, forming the Betic-Rif Massif, and the Tirrenian region formed a continuum with the Iberian peninsula, the Italian peninsula, the South of France, the North of Africa and the Macaronesian region. During this so dry period Asplenium azomanes colonized the limestone oriented to the north and northwest of that vast region. The Messinian period lasted one million years and, then, the water level of the Mediterranean Sea rose again, when the European and African tectonic plates separated and formed a great groove, the Strait of Gibraltar, which allowed the water inlet of the Atlantic Ocean to the Mediterranean Sea and the population of the Asplenium azomanes was fragmented into three regions: the Balearic Islands (Mallorca, Ibiza and Formentera), that no longer mountains and became islands, south of the Iberian peninsula (Cadiz, Malaga, Albacete, Jaen, Murcia and Almeria) and northern Morocco (from Chefchaouen in Western Rif until the rocks surrounding the Caves of Chameau in Mountains of Beni Snassen).

 

 

It is typical of the Asplenium azomanes that the medium and lower pinnae have a small auricle at the base of each pinna directed toward the apex of the frond, with one or two sori inside the auricle. The phenotype of Asplenium azomanes change much as the habitat where they live. If grown in a sunny spot has the pinnae large, bright, convex, and very very coriaceous boards, as shrinkage on the rachis, prepared at various levels as a roof. On the contrary, if it grows into a more somber habitat, has the pinnae smaller and rounded, softer touch, very separated from each other and arranged in the same plane. What remains unchanged is the rachis which is always very thick, dark garnet color and bright. From this ancestral hybrid have formed two allopolyploid hybrids: a allotetraploid hybrid in the Balearic Islands, the vigorous hybrid Asplenium X tubalense ( for the german botanists it is the Asplenium trichomanes nothosubsp. barreraense ), product crosses between Asplenium azomanes and Asplenium trichomanes ssp. quadrivalens and a hexaploid hybrid in Andalusia,  Asplenium  trichomanes nothosubsp. malacitense, by crossbreeding with Asplenium trichomanes ssp. inexpectans ( I found this hexaploid in a torrent very bleak of Cortes de la Frontera, province of Malaga, in the Natural Park of Los Alcornocales).

 

Asplenium anceps

         v

         v

         X< < < < < < < <  Asplenium ...............?

         v

         v

         v

         v

         v

Asplenium azoricum

         v

         v

         X< < < < < < < < Asplenium trichomanes ssp. hastatum ?

         v

         v

         v

         v

         v

Asplenium azomanes

       v

         v

         X< < < < < < < <  Asplenium trichomanes ssp. quadrivalens

         v

         v

         v

         v

         v

Asplenium X tubalense

 

The Asplenium trichomanes ssp. quadrivalens, autotetraploid, besides hybrids mentioned above, has produced two more hybrid ferns: the Asplenium trichomanes nothosubsp. lucanum, triploid hybrid by crossing with the Asplenium trichomanes ssp. inexpectans (diploid) and the Asplenium X helii nothosubsp. lainzii, allotetraploid, by hybridization with the Asplenium petrarchae ssp. petrarchae (autotetraploid), which is much like its cousin the Asplenium X orellii, but, unlike this one which is glabrous, the Asplenium X helii nothosubsp. lainzii has multicellular trichomes or glandular hairs on the petiole and proximal part of the rachis, many of which are branched.

 

Asplenium petrarchae ssp. bivalens

Asplenium fontanum

Asplenium petrarchae ssp. petrarchae

Asplenium trichomanes ssp. quadrivalens

Asplenium trichomanes ssp. inexpectans

Asplenium trichomanes nothosubsp. malacitense

(Andalusia)


There are more hybrids, most very little (sometimes their total population does not exceed two or three copies throughout the Serra de Tramuntana) and very difficult to distinguish, so I limited my exposure to which I have found and photographed.

 

 

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