Undaria pinnatifida is the commercially second most important brown alga in the world. Its global annual yield has been more than two million tonnes since 2012. It is extensively cultivated in East Asia, mainly consumed as food but also used as feed for aquacultural animals and raw materials for extraction of chemicals applicable in pharmaceutics and cosmetics. Cultivar breeding, which is conducted on the basis of characteristics of the life history, plays a pivotal role in seaweed farming industry. The common basic life history shared by kelps determines that their cultivar breeding strategies are similar. Cultivar breeding and cultivation methods of U. pinnatifida have usually been learned or directly transferred from those of Saccharina japonica. However, recent studies have revealed certain peculiarity in the life history of U. pinnatifida. In this article, we review the studies relevant to cultivar breeding in this alga, including the peculiar component of the life history, and the genetics, transcriptomics and genomics tools available, as well as the main cultivar breeding methods. Then we discuss the prospects of cultivar breeding based on our understanding of this kelp and what we can learn from the model brown alga and land crops.

The brown alga Undaria pinnatifida (Harvey) Suringar is an economically important kelp species native to the Northwest Pacific and has been extensively farmed as human food in East Asia for more than half a century (Yamanaka and Akiyama, 1993). It is also an important resource for extracting biologically active compounds such as fucoidans which have diverse applications in pharmaceutical and cosmetic industries (Zhao et al., 2018; Yoo et al., 2019). Its annual yield worldwide has been more than two million tons since 2012 (http://www.fao.org/fishery/species/2777/en). Nowadays U. pinnatifida has become a cosmopolitan species due to its worldwide spread in recent decades, attracting increasing public attention (South et al., 2017). It has been listed as one of the world's 100 worst invasive species (Lowe et al., 2000), and in Europe has been regarded as one of the top 10 worst invasive species (Gallardo, 2014).

Sporophyll development is an important biological process which is crucial to reproduction of the sporophyte of the brown alga Undaria pinnatifida. However, the molecular foundation underlying this process is largely unknown due to lack of genomic information in this important alga. Full-length transcriptome, which covered male and female gametophytes at vegetative and gametogenic phases, and the blade, stipe, immature and mature parts of the sporophyll of sporophytes, was obtained for the first time through single molecule real-time sequencing in this study. After removing potentially polluted sequences derived from microorganisms, we obtained a total of 61,181 transcripts with average and N50 length being 1639 and 1815 bp, respectively, and annotated 21,915 of them (35.8%). By using the full-length transcriptome as reference, we performed Illumina sequencing to analyze the difference of gene expression among different developmental stages of the sporophyll and validated the results by quantitative real-time PCR. A large number of transcripts related to flagellar components and fucoidan biosynthesis as well as a few transcripts coding for meiotic nuclear division protein were found to be significantly upregulated with development of the sporophyll, and thus the over-expression of them is likely essential for the sporophyll to perform its biological function. These findings contribute to our understanding of sporophyll development in U. pinnatifida at the transcriptional level.

Undaria pinnatifida, a kelp species native to East Asia, has become cosmopolitan and drawn increasing attention due to its worldwide spread in recent decades. Floating fragments of this alga were found washed ashore on Sylt in 2016, the first record of this species in Germany. Thalli attached to local oyster reefs were detected in 2017. The genetic relationship between the floating and attached thalli on Sylt, as well as their relevance to the populations from northern Europe and native regions, was hitherto unknown. Here, 10 microsatellite markers were used to assess relationships between the recently established population on Sylt and five other northern European populations in France (Brittany, West English Channel), the Netherlands and England (Plymouth, West English Channel) plus three natural populations in China. Almost no genetic differentiation was detected between the floating and attached populations on Sylt, but they were genetically distinct from all the other studied northern European populations. The very low genetic diversity revealed in the new founder populations of Sylt suggests that they came from genetically similar parents. The marked reduction in both the number of alleles and heterozygosity in the northern European populations, as compared with the Chinese ones, is typical of founder effects in recently populated regions. Prominent genetic divergence was found between most of the northern European populations except those within Brittany and Sylt. Further studies will focus on identifying the putative source populations that might be found on shellfish farms, in local marinas or the benthic habitats around Sylt Island.

Hermaphroditic gametophyte production and an intact male life cycle were investigated in Undaria pinnatifida. Zoospore-derived hermaphroditic gametophyte could give rise to male sporophytes which showed the same morphological appearance but with smaller biomass than normal cultivated sporophytes. Male sporophytes were shown to discharge only male zoospores. In this investigation, male spores derived from the male sporophytes and female spores derived from female parthenosporophytes were crossed to produce the hybrid sporophytic offspring. The latter contained both alleles of homozygous parental sporophytes when analyzed by use of eight polymorphic microsatellite markers, further confirming the success of cross. Thus, line crossing by use of unisexual sporophytes will be expected to replace unisexual gametophyte line crossing in breeding practice in the future in this commercially farmed seaweed.

Undaria pinnatifida is an important economic brown alga in East Asian countries. However, its genetic and genomic information is very scarce, which hinders further research in this species. A high-density genetic map is a basic tool for fundamental and applied research such as discovery of functional genes and mapping of quantitative trait loci (QTL). In this study the recently developed specific length amplified fragment sequencing (SLAF-seq) technology was employed to construct a high-density genetic linkage map and locate a sex determining locus for U. pinnatifida.

The annual life cycle of the brown seaweed Undaria pinnatifida (Harvey) Suringer comprises a macroscopic diploid sporophyte stage and a microscopic haploid gametophyte stage. In 2011, an unusual zoospore-derived monoecious gametophyte isolate (designated as line 10-5-3) of U. pinnatifida was observed. To understand this phenomenon, a comprehensive screening of eighty-two previously identified male gametophyte cultures, isolated from three randomly selected cultivars (lines 10, 7, and 5) was performed. Thirty-six of the isolates developed both antheridia and oogonia on the same filamentous fragment in a standard gametogenesis test (SGT: 18°C, 60 μmol photons/(m2∙s)). Selfing of the monoecious gametophyte or crossing it with a normal male gametophyte both gave rise to morphologically normal sporophytic offspring. However, crossing resulted in a much higher fertilization rate (89.7%). The hybrid and selfed sporophytic offspring were grown to maturity in flow tanks at an ambient temperature of 10–18°C over a period of 69 days. Active zoospores were released from both types of mature sporophylls. The majority of these developed into male gametophytes, while 15%–20% developed into the observed monoecious structures on the same filament. Using PCR amplification it was found that all the monoecious gametophyte isolates and the sporophytic offspring resulting from the selfing and crossing lacked the femalelinked microsatellite sequence (a part of the locus Up-AC-2A8, GenBank accession No. AY738602.1), indicating their male nature. U. pinnatifida is an invasive species in some regions and the implications of the above findings for this species in nature are briefly discussed.