Sexual reproduction is widespread, but asexual lineages have repeatedly arisen from sexual ancestors across a wide range of eukaryotic taxa. The molecular changes underpinning the switch to asexuality remain elusive, particularly in organisms with haploid sexual systems. Here we explore independent events of loss of sex in the brown alga Scytosiphon, examine the proximate and evolutionary mechanisms involved, and test the importance of sexual conflict on gene expression changes following loss of sex. We find that asexual females (‘Amazons’) lose ability to produce sex pheromone and, consequently, are incapable of attracting males, whereas they gain rapid parthenogenic development from large, unfertilized eggs. These phenotypic changes are accompanied by convergent changes in gene expression. Decay of female functions, rather than relaxation of sexual antagonism, may be a dominant force at play during the emergence of asexuality in haploid sexual systems. Moreover, we show that haploid purifying selection plays a key role in limiting the accumulation of deleterious alleles in Amazons, and we identify an autosomal locus associated with the Amazon phenotype. The sex chromosome, together with this autosomal locus, may underlie the switch to obligate asexuality in the Amazon populations.

有性繁殖很普遍，但无性谱系反复出现于广泛的真核生物分类群的有性祖先。支撑转向无性恋的分子变化仍然难以捉摸，尤其是在具有单倍体性系统的生物体中。在这里，我们探讨了褐藻 Scytosiphon 中性行为丧失的独立事件，研究了所涉及的近因和进化机制，并测试了性冲突对性行为丧失后基因表达变化的重要性。我们发现无性雌性（“亚马逊”）失去了产生性信息素的能力，因此无法吸引雄性，而它们从未受精的大卵中获得快速的孤雌发育。这些表型变化伴随着基因表达的收敛变化。女性功能的衰退，而不是性对抗的松弛，可能是单倍体性系统中无性恋出现过程中起作用的主导力量。此外，我们表明单倍体纯化选择在限制 Amazon 中有害等位基因的积累方面起着关键作用，并且我们确定了与 Amazon 表型相关的常染色体基因座。性染色体与这个常染色体位点一起可能是亚马逊人群转向专性无性恋的基础。