Wheaton College, Department of Biology, Wheaton, IL 60187
The kelps, brown algae of the order Laminariales, are marine multicellular organisms that reproduce by alternation of heteromorphic generations. During the reproduction cycle, sexual zoospores are produced by meiosis and are released from the large mature sporophytes. Spores then germinate into microscopic male or female gametophytes by mitosis. Sexuality of gametophytes is determined genetically. Female gametophytes produce sessile eggs while male gametophytes produce motile sperm. As male and female gametes fuse, a zygote is formed, which grows into an adult sporophyte. Several factors influence gametogenesis in kelp gametophytes, including blue light, iron nutrition, and salinity. This experiment was designed to investigate the effect of salinity level on oogenesis of two ecotypes of Alaria marginata, one of typical A. marginata and the other formerly referred to as A. tenuifolia. Ecotypes refer to those organisms which exhibit genetic differences in morphology and/or physiology and yet are not distinct enough genetically to be categorized into different species. Based on the genetic studies of the genus Alaria, it was recommended to regard what is known as Alaria tenuifolia to be a synonym of A. marginata due to lack of genetic distinctions. For this study, A. marginata was collected from the outer coast of Vancouver Island where salinity is relatively consistent and high (28-31 psu) while the “A. tenuifolia” form was collected from the inner coast near the city of Vancouver where salinity fluctuates between 20 and 28 psu throughout the year due to nearby river run off. To determine the effects of salinity level on oogenesis as well as optimal salinity ranges, eight different salinity treatments ranging from 12 to 33 psu were applied to isolated female gametophytes of both strains of A. marginata. The number of eggs produced by 50 gametophytes in each of 6 replicates was counted after 14 days. These gametophytes produced eggs at all salinities tested. Varying salinity significantly affected the quantity of oogenesis of both ecotypes in A. marginata (F_7,40=268.9,p<0.0001) and former A. tenuifolia (F_7,40=127.3,p<0.0001). Gametophytes of the typical A. marginata form produced the most eggs at 27 psu and the ”A. tenuifolia” form between 18 and 21 psu. Even though they are considered to be the same species, they show evidence of ecotypic differentiation in relation to their response to the salinity of the seawater in which they live. Further studies are planned to explore the responses of spermatogenesis and growth of young sporophytes of these same strains to varying salinity.
J. Jo was supported by the Wheaton College 2012 Summer Research Program.
[Abstract (DOCX)]