Growth characteristics of hybrids produced by closely related Ulva species

Comparative studies of closely related species from different habitats can potentially elucidate mechanisms of adaptation. Hybrids can act as valuable archives, enabling the evaluation of speciation. Brackish Ulva prolifera and marine Ulva linza show partial gamete compatibility and have been identified as closely related Ulva species based on molecular phylogenetic analysis. However, there are few reports on the biological characteristics of hybrids produced by brackish U. prolifera × marine U. linza. This study aimed to compare the morphological and growth characteristics of these two closely related Ulva species and their hybrids. The results showed that hybrid sporophytes had 1.0–3.6 branches per 1 cm of main stem, and the hybrid gametophytes had multiple branching characteristics, including the unbranched flat U. linza type, branched tubular U. prolifera type and branching types that were intermediate between the two. In addition, brackish U. prolifera, marine U. linza and their hybrid sporophytes exhibited high germination rates, approximately 6.6 times greater than hybrid gametophytes (12.7%). The highest daily growth rates of brackish U. prolifera and marine U. linza were 179.2% and 172.7%, respectively, which were significantly higher than those exhibited by hybrids (153.3%). These results demonstrated that the closely related brackish U. prolifera and marine U. linza have a greater capacity to adaptive to their environment than hybrids of the two species.