## 6 The Genetic Basis of Sexual Reproduction in _Haslea_  
The number of identified representatives of the marine genus _Haslea_ continuously increases. Thirty-six taxonomically accepted species are currently listed in the AlgaeBase [45]; however, sexual reproduction has been observed in only five of them, including one non-blue- and four blue-colored ones. It should be mentioned that sexual reproduction and auxosporulation is an obligate stage of the life cycle of the vast majority of diatoms-- due to the specific construction of the cell frustule, predetermining its diminution as a result of vegetative divisions--and subsequent restoration of the maximal size during sexual reproduction [111, 112]. In unicellular organisms, including diatoms, mating can occur between gametes produced within the same strain (homothallic mating system) or between gametes produced by strains of complementary mating types (heterothallic mating system). [PERSON] and [PERSON] were the first to describe sexual process in _Haslea_ (_Navicula_) _ostrearia_[113, 114]. However, that was intraclonal (homothallic) reproduction, and nobody could repeat their experiments for a long time. Thirty-six years later, heterothallic sexual reproduction was described in this species and heterothally was demonstrated as dominant in the breeding system, whereas intraclonal reproduction is rare and sporadic [31]. Heterothallic sexual reproduction was also observed in a second blue _Haslea_, _H. Karadagensis_, newly described after its reproductive isolation from _H. ostrearia_ was identified [36, 115]. Subsequently, the life cycle and sexual reproduction of _H. provincialis_[39] and _H. silbo_[41] were studied. So far, sexual reproduction has been described in only one non-blue _Haslea_: _H. subagnita_[116]. A highly species-specific, finely organized scheme of the auxosporulation process in diatoms requires the manifestation of sexes, which, in most centric species, is regulated  
Figure 6: Results of ELISA tests performed using _H. ostrearia_ exposed to (**a**,**b**) ultraviolet (UV) stress or (**c**,**d**) diuron treatments. In both conditions, cells were grown under 100 μmol photons m\({}^{-2}\) s\({}^{-1}\), and under 14 h/10 h L/D cycles. The 2 h UV radiation (UVR) stress consisted of UVB (450 mW m\({}^{-2}\)) + UVA (11 W m\({}^{-2}\)) exposition. Diuron was used at different concentrations (0; 5 and 10 μ/L). Graphs 5 mC and 5 hmC correspond to the ELISA assay quantifying the global level of C-methylation and active demethylation of the genome, respectively. epigenetically; meanwhile, in pennates, it is determined genetically and does not change throughout the entire life history of a particular clone. For this reason, most pennate diatoms are diclinous, and cells of two opposite or complementary sexes are needed for sexual process to be triggered [112]. The exceptions are occurrences of paedogamy and automixis. Until recently it was unclear how the transition from the vegetative phase to sexual reproduction is regulated at the molecular level. The attempt to identify sexual-reproduction-specific genes in diatoms was first undertaken a little more than two decades ago [117]. It was found that multiple genes are upregulated during the onset of sexual reproduction in _Thalassiova weissflogii_ ([PERSON]) [PERSON] and [PERSON], currently regarded as _Conticribra weissflogii_ ([PERSON]) [PERSON] and [PERSON] [118]. A novel gene family, referred to as _Sig_ genes, has been shown to be expressed during sexual reproduction. Ten sexually inducible genes have been identified, although it was very likely that they represented only a small fraction of all sexual-reproduction-specific genes in _T. weissflogii_[117]. The preliminary stages of gametogenesis may involve complex (in some species) cascade pheromone activity [119; 120; 121; 122], which implies the successive activation of different genes. Evidence for the presence of sex pheromones was provided for several species, all of them being pennate.