## 6 The Genetic Basis of Sexual Reproduction in _Haslea_
They demonstrated a predominantly heterothallic mode of reproduction. Multi-stage gene expression presumably allows vegetative cells to exit the mitotic cycle and undergo meiosis to form gametes [117]. Each of the two mating types (hereafter, MT) secretes specific pheromones, triggering MT-specific responses. Interaction via pheromones attracts sexual partners, then stops their cell cycle and synchronizes a switch from mitosis to meiosis and gametogenesis. The pheromones involved in cell cycle arrest are chemically distinct from those used to attract the opposite MTs [121]. Apart from _S. robusta_, involving multiple signal molecules has been demonstrated for _P. multistriata_[121]. A less complex pheromone system was reported for _Cylindrotheca closerium_ ([PERSON] [PERSON] and [PERSON], where no evidence of MT+ producing pheromones was found [122]. Until recently, the genetic basis of sex determination was not known for diatoms. Various types of sex determination systems have evolved in different eukaryotic groups [123; 124]. For diocious species, the sex chromosomes are referred to as XY or ZW, depending on which is the heterogametic sex. Sex can also be determined by environmental cues (epigenetic regulation in the case of centric diatoms), by the implication of multiple genes or of cytoplasmic elements. In contrast to epigenetic regulation in centric diatoms, in pennate species, sex factors (designated as M and F) were shown to be inherited independently, and the combinations MF and FF determine male (heterogametic) and female (homogametic) descendants, respectively [125; 126; 127; 128]. The gametes of the investigated species could be differentiated by their behavior and mode of formation, which allows the application of the terms \"male\" and \"female\". Later, the mating type plus (MT+) was confirmed as the heterogametic sex in _S. robusta_[129]. In a study aimed at comparing the expression profiles of the two MTs in _P. multistriata_, five mating-type-related genes were found, of which three were Mating-type Related Plus (MRP) and two were Mating-type Related Minus (MRM). One of the MRP genes seems to be determined by a single mating-type locus [130]. A system based on a single-gene sex determination could potentially have evolved in diatoms. However, more reliable is a system suggesting that two genes are involved, because three genders (males, females and hermaphrodites) may be controlled by a single genetic locus [131; 132]. In some pennate diatoms, such as for _H. ostrearia_, bisexual clones also occur, thus revealing the existence of both dioocy and monoecy [133]. Genomic regions associated with sex determination are often characterized by the lack of recombination, which results in reduced efficacy of selection against structural mutations [134]. Theoretically this promotes the creation of chromosomal heteromorphism and determines the size differences between sex chromosomes [132]. It can be noted, however, that dimorphic sex chromosomes are unknown in diatoms. The same is observed in vascular plants, where sex chromosomes have been identified in very few species so far. Among dioecious plants with cytogenetic and/or molecular evidence for the presence of sex chromosomes, male heterogamety (XY) is predominant, while female heterogamety (ZW) is very rare [135]. Shifts between different sex chromosome systems (XY vs. ZW) are also well documented and appear to be a common phenomenon. Sexual systems may differ, even in representatives of the same genus [135]. In some plants, the Y-specific suppressor of female function (SOFF) gene acts as a suppressor of femaleness. Experimental gamma irradiation knockout resulted in the conversion of males to hermaphrodites [136]. Given this case, we can hypothesize that the same type of sex regulation is at work in dioecious, predominantly heterotrophic diatoms capable of rare intraclonal reproduction. Fairly close relatives to diatoms among Stramenopiles, for which information is available regarding life cycles and breeding systems, are the multicellular brown macroalgae.