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PHYCOLOGICAL SOCIETY OF AMERICA
Pugetia fragilissima & Pugetia firma
Life Cycle
Reproductive mechanisms are key to the evolutionary and ecological success of all organisms. Different species of algae demonstrate a wide array of specialized strategies for assuring reproductive success despite the challenges presented by their ocean medium. This diversity is particularly pronounced in Rhodophyta, a division known for reproductive variations. Both Pugetia firma and Pugetia fragilissima appear to have a relatively typical red alga life cycle, but unfortunately very little research has been conducted on the specifics of their life cycles. The limited information available has been derived from general Rhodophyta reproductive patterns, one study in which Pugetia firma was grown in culture, and observations made during this study. Due to the lack of specific data, many possible reproductive mechanisms seen in other red algae are considered here, but more substantive research is needed before any specific conclusions can be established.
Like other red algae, Pugetia demonstrates a haplodiplontic triphasic life cycle, meaning it alternates between three life stages with varying ploidy levels. The three life phases include a haploid gametophyte generation and a diploid tetrasporophyte generation with a reduced parasitic carposporophyte generation connecting them. All detailed information on the life history of Pugetia comes from a 1972 study, in which Steven Murray and Peter Dixon followed the complete life cycle of Pugetia firma (then known as Callophyllis firma) in culture (Murray and Dixon, 1973). It should be noted that although it is probably safe to assume that the life histories of Pugetia firma and Pugetia fragilissima are similar, the differences in their reproductive structures should prevent generalization about Pugetia fragilissima based on the Pugetia firma culture studies. Furthermore, environmental conditions can have effects on morphological phenotypes and may not be clearly demonstrated through studies of algal cultures (Lobban and Harrison, 1997).
Murray and Dixon collected gametangial plants bearing mature carposporophytes. These samples released diploid carpospores, which germinated, producing small-multilayered basal discs, from which erect axes arose and developed into young cartilaginous thalli. The resulting tetrasporophytes matured and approximately two months later produced haploid tetraspores in cruciate arrangements from the inner cortical cells. These tetraspores can be seen through a simple compound microscope dispersed along the entire blade. Once released, these tetraspores germinated, growing into gametangial plants that were morphologically identical to the tetrasporophyte.
Eventually the female gametophytes developed mature carposporphytes, indicating that male gametophytes had released spermatia, which had been successfully captured by the female trichogyne (a carpogonial extension that may serve as a “fishing rod” for spermatia). Unfortunately, male gametophytes were not identified in either Murray and Dixon’s study or this one, presumably due to the minuscule size and distribution of the spermatangia. It is possible that gametangial plants are dioecious, producing both female and male gametes. More research is needed before anything can be stated definitively. Either way, it remains clear that spermatia reached female trichogynes and fertilized the carpogonia (eggs). The nuclei of these zygotes would have been transported to auxiliary cells where they would have mitotically divided, producing carpospores, aggregated in protective cystocarps. This entire cycle was shown to occur in as little as six months.
Alternation of generations allows Pugetia and other red algae to undergo complete its sexual cycle. Reproduction allows for large population increases with little energy expenditure. For red algae, this often takes the form of zygote amplification during the formation of carpospores, which was believed to have evolved in Rhodophyta to compensate for their non-motile spermatia (Lobban and Harrison, 1997). Crucial genotypic variation within the population is acquired through meiosis and sexual recombination, however this strategy can be very costly when gametes are lost in the water column. Like other reds, Pugetia has partially overcome this problem through oogamy, and the retention of its non-motile female gamete or egg cell. By "brooding" carpogonia, Pugetia reduces the number of gametes wandering through the water column.
To further ensure syngamy (fertilization), the female gametophyte must have effective spermatia collecting mechanisms. The trichogyne on the tip of the carpogonial branch may gather spermatia from the surrounding water. Considering the spermatia of red algae lack flagella and thus have limited mobility, further mechanisms are critical to ensure contact between male and female gametes. Some species have been shown to produce mucilage strands that stretch from male to female gametophytes, transferring spermatia (Lobban and Harrison, 1997). Although such a strategy has never been confirmed in Pugetia, on two different occasions I observed mucus strands extended between Pugetia fragilissima thalli clumped on the same tubeworm. However, it is possible that the mucus was part of the feeding bags extended by the polychaete worm itself.
Whether the spermatia traveled by mucus or not, more than likely, they did not travel particularly far. Individual plants from all life stages were found aggregated on the same small substrate whether it was a tubeworm, a rock face or another red alga. These small Pugetia “colonies” were often isolated by distances greater than 10 m from each other and thus may be relatively insular with their reproduction. It is may be that spermatia fertilized neighboring females and that carpospores and tetraspores had limited dispersal. It would be very interesting to use molecular sequencing to decipher what level of gene flow occurs between these populations, or even between weeds growing on different tube worms.
Synchrony of gamete production and release would be crucial for successful syngamy and to prevent gamete loss. This coordination would require some sort of environmental cue such as temperature, light, salinity or turbidity changes and would probably occur seasonally. However, between samples found in the G.M. Smith Herbarium at Hopkins Marine Station and those observed during this study, all stages of the Pugetia life cycle have been documented all year long. With an unclear seasonal growth and reproductive pattern, it would be difficult to determine which environmental cue triggers gamete production and the release of spermatia. Until more research can be conducted, all we can determine with certainty is that Pugetia firma and Pugetia fragilissima have been successful reproducing and maintaining populations along the coast of the Monterey Peninsula for over fifty years.
© 2005 Laure Sierra Katz.
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If you have any questions, comments or happy ocean stories
please email me at laurek@stanford.edu