SEED MORPHOLOGY, POLYPLOIDY AND THE EVOLUTIONARY HISTORY OF THE EPIPHYTIC CACTUS RHIPSALIS BACCIFERA (CACTACEAE)

Abstract

A SEM survey of seed, stem, stomata, and
fruit characters was conducted to investigate patterns of infraspecific variability in
Rhipsalis baccifera. New and Old World
seeds were analyzed to assess the taxonomic
value of their morphological features and
the presence of gigas characters in polyploid
versus diploid subspecies. The seeds are
mussel-shaped and correspond to the Rhipsalis-type. Old World representatives have
primarily oval seeds, whereas narrowly oval
to oval seeds are more common in New
World accessions. The seed coat is glossy,
smooth, and without secondary sculpturing.
The cell outline is slightly irregular with an
overall elongate to rectangular shape. Cell
size increases from the hilum-micropylar
region to the apical portion of the seed.
Seed and cell size increase with increasing
level of polyploidization, its maximum
expression occurring in polyploid African
populations. It is likely that increase in seed
size in the Old World is correlated with polyploid cytotypes. An increase in stomatal
cell size is not evident with an increase in
chromosome number, though stem and fruit
size and the number of stomata are higher in
the Old World polyploid subsp. horrida. The
existence of smaller seeds in Paraguay and
northern Argentina suggests that this South
American region is the center of origin of R.
baccifera, from where it radiated to North
America and the Old World via eastern
Brazil. We hypothesize that the extensive
geographic distribution of R. baccifera in
the New and Old Worlds has been possible
due to reproductive strategies, progressive
and recurrent cycles of polyploidy and dispersal events by migratory birds.