Experimental hybridization of Palmaria palmata (Rhodophyta) from the northeast and northwest Atlantic Ocean

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Abstract
Isolates of Palmaria palmata from the Canadian Maritimes and isolates of two varieties of P. palmata from Ireland (var. palmata and var. sobolifera) were crossed in pairwise combinations within and between these geographic locations. Crosses between plants from a single location always yielded normal tetrasporophytes and normal F1 gametophytic progeny; however, within the Irish group there was a complex phenotypic segregation for gametophytes from var. palmata × var. sobolifera hybrids. Robust "transatlantic" hybrids were obtained without difficulty in crosses between gametophytes from Canadian and Irish isolates. These hybrids had normal morphology and vigorous growth; however, their tetraspores were poorly viable, and those that germinated produced a variety of abnormal sporelings. In sporangia of the hybrid tetrasporophytes, the meiotic chromosomes were tangled in complex arrangements suggestive of multiple heterozygous chromosome rearrangements. Despite the complex chromosome pairings and abnormal sporelings, some genetic recombination was observed between a Mendelian mutation for green color and the sexual phenotype of the plant. It is concluded that the Irish and Canadian plants belong to a single species, but one that is in the process of splitting into sibling species as a consequence of geographic separation and the diverging karyotypes of its constituent populations.

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