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PHYCOLOGICAL SOCIETY OF AMERICA
The life cycle shown below illustrates the triphasic, isomorphic life history of Gelidium. The three life stages, gametophyte, carposporophyte and tetrasporophyte, and the indistinguishable external morphology of the gametophyte and tetrasporophyte stages characterize this life history as the Polysiphonia-type. Reproductive characteristics are used for classification of the Rhodophyta, but much debate continues in defining and differentiating these structures.
The haploid male gametophyte releases non-flagellated spermatia that will fertilize the carpogonia attached to the haploid female gametophyte. Each carpogonium contains a hair-like receptive structure known as a trichogyne. This structure is involved in receiving the spermatium. Nutritive filaments develop during the formation of the carpogonia, before fertilization. When a carpogonium is fertilized by a spermatium, the resulting growth is the diploid intermediate life phase known as the carposporophyte. This phase grows attached to the female gametophyte in a structure called a cystocarp. The cystocarp and carposporophyte both develop after fertilization has occurred. The tissue of the cystocarp is haploid while the tissue of the carposporophyte inside is diploid. A cross section of a cystocarp and carposporophyte under a microscope is shown below.
The carposporophyte releases diploid carpospores that settle
and grow into diploid tetrasporophytes. As Gelidium spores germinate,
they undergo a characteristic process in which the contents of the spore
are transferred to a germ tube and large vacuoles are formed in the spore
cell, leaving it apparently empty. The tetrasporophyte undergoes meiosis
in its reproductive areas to create haploid tetraspores that are cruciate
in Gelidium. When these tetraspores settle, the tetrad breaks up
and each spore grows to become the haploid male and female gametophytes.
Recent field studies of Gelidium species have introduced questions as
to whether all Gelidium exhibits the Polysiphonia-type life history
without any modifications. In the Polysiphonia-type life history, gametophytes
are either male or female, described as dioecious, and an equal ratio of gametophytes
and tetrasporophytes should exist in a population at any given time. Field studies
of Gelidium have revealed that monecious gametophytes are found in some
species where spermatangial patches and carpogonia occur on the same branch.
Also, many studies of reproduction in wild populations show an unequal ratio
of tetrasporophytes to gametophytes. Since more tetrasporophytes are ususally
found, it is thought that some tetrasporophytes may not undergo meiosis, and
may in fact release mitotically divided diploid spores that grow into more tetrasporophytes.
If this is the case, a side loop would be introduced to the life history. More
investigation must occur before conclusive results can be determined on this
topic.
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© 1999 Sharon C. Komarow. All rights reserved.