Codominant inheritance of polymorphic color mutant and characterization of a bisexual mutant ofGracilaria caudata(Gracilariales, Rhodophyta)

The red algaGracilaria caudatais one of the main agarophytes exploited on the Brazilian coast. A greenish-brown mutant was found growing side-by-side with the common red wild type in a natural population. In this work, we tested the hypotheses that the greenish-brown phenotype has a recessive nuclear inheritance and less fitness (growth, photosynthesis, number of apices, secondary attachment percentage, and number of cystocarps) under laboratory conditions when compared with the red phenotype. Crosses between plants of the same color (red or greenish-brown) showed tetrasporophytic and subsequent gametophytic generations with parental phenotype. However, heterozygous tetrasporophytes, obtained from crosses between plants with different colors, showed a brown phenotype. From their progeny, we concluded that the brown phenotype is due to a monolocus Mendelian genetic system with two codominant alleles. Bisexual gametophytes were found in the progeny of tetrasporophytic color mutants, which is an unusual and poorly known characteristic in the life cycle of Gracilariales. Red female gametophytes (rdgenotype) presented better performance in relation to the greenish-brown female gametophytes (gbgenotype): growth rates (rd = 17.14 +/- 1.36,gb = 13.81 +/- 0.95); number of apices (rd = 41.05 +/- 6.35,gb = 16.00 +/- 5.47); secondary attachment percentage (rd = 68.75 +/- 23.93,gb = 25 +/- 0.00); and number of cystocarps (rd = 38.61,gb = 18.81). Our results explain the maintenance of the greenish-brown mutant and the predominance of red specimens in natural population. The life cycle was completed in 5 months, one of the shortest times reported forGracilaria, highlighting the importance of the species as a possible model organism for reproductive studies and in selecting strains for use in mariculture. (AU)