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(Excerpted from Hickok, Warne and Fribourg, 1995)

Ceratopteris is a genus of homosporous ferns found in most tropical and subtropical areas of the world. Species grow as either aquatics or sub-aquatics and are limited in habitat to ponds, rivers or other wet areas such as ditches, taro patches or rice paddies. Although some require an aquatic habitat, most species can be successfully grown in pot culture in the greenhouse under warm, humid conditions. Ceratopteris is eaten in areas of Southeastern Asia and there was an early attempt to develop it as a crop in the Philippines. Currently, commercial uses are primarily limited to its widespread sale as an aquarium plant in fish hobbyist stores, where it is sold under the common name of 'water sprite' and has even been immortalized in plastic replicas.

Instead of the long-lived rhizome and perennial nature that characterizes most homosporous ferns, Ceratopteris possesses a short upright rhizome and grows as an annual. The fronds are dimorphic. Initial fronds are sterile and simple to 3-lobed but as development proceeds the fronds become increasingly more dissected and fertile. Sporangia are found in continuous rows on veins along the ventral edges of fertile fronds and are covered by the inrolled margin of the frond, forming a false indusium. In addition to sexual reproduction by meiotic production of spores and subsequent gametophytes, Ceratopteris sporophytes have a prolific capacity for vegetative reproduction. Buds found in the axes of subdivisions of the frond can develop rapidly into plantlets. Both in nature and under greenhouse culture, it is not unusual to find senescing fronds covered with developing plantlets that originated from these buds. The buds are a convenient means of vegetatively propagating particular genotypes.

Because Ceratopteris shows a variety of morphological characters that are exhibited by supposedly unrelated and related families, phylogenetic affinities have been uncertain. While some treatments have placed Ceratopteris in its own monotypic family, Parkeriaceae, others have included it as a subfamily or tribe within the Pteridaceae. Taxonomic treatments by Robert M. Lloyd (1974, 1993) recognized four species: C. thalictroides, C. richardii, C. pteridoides and C. cornuta. Ceratopteris thalictroides is described as a highly polymorphic species with pan-tropical distribution. Ceratopteris pteridoides and C. cornuta, show some similarities to each other but are both morphologically distinct from C. thalictroides. Ceratopteris pteridoides is principally limited to Central and South America while C. cornuta is mainly confined in distribution to mainland Africa. The fourth species, C. richardii, is morphologically similar to C. thalictroides; the only consistent difference noted by Lloyd (1974, 1993) was the 16-spored sporangia of C. richardii as opposed to the 32-spored sporangia of the other species. Cytological observations show that C. thalictroides is tetraploid with n = 77,78 and that the other three species recognized by Lloyd are diploid with n = 39. Analyses of synthesized interspecific hybrids indicated incomplete reproductive isolation among the diploid species. The presence of herbarium specimens with both 16 and 32-spored sporangia and exhibiting mixed features of both C. richardii and C. pteridoides suggest that hybridization occurs naturally (Lloyd 1993).

Strains

Although a number of homozygous diploid and tetraploid accessions of Ceratopteris are available, the Hn-n strain has been used in most experimental studies. Hn-n is a diploid (n = 39) derived from a collection of C. richardii from Cuba (Killip 44595 GH). In addition, two additional diploid strains, 176D and phiN8, are also in current use (see Hickok, Warne and Fribourg, 1995). All three strains show sexually dimorphic gametophytes, i.e., males and hermaphrodites, with males developing in response to the pheromone, antheridiogen (A_Ce). However, 176D shows only slight sensitivity to antheridiogen as compared to Hn-n and phiN8 and consequently has few males in multispore cultures. The three strains can be crossed readily. Hybrids between Hn-n and 176D show ca. 90% spore viability while crosses of phiN8 with either Hn-n or 176D show ca. 30% spore viability.

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