Facultative parthenogenesis in an asexual stick insect species: re-evolution of sex or vestigial sexual capacity?

Among the diversity of reproductive modes that are known in nature, most animal species use a pure strategy: either obligate sex or obligate parthenogenesis. In comparison, mixed strategies are relatively scarce. Here, we describe a case of facultative parthenogenesis in the North American stick insect Timema douglasi, a species in which only all-female populations were known so far. We found extensive population sex ratio variation at a small geographic scale, but females from all populations were capable of reproducing via automictic parthenogenesis. Genotyping by sequencing revealed the presence of three genetically differentiated lineages. Females of these lineages differed in their propensity to use sexual reproduction when mated, with largely or exclusively sexual reproduction in one lineage, and largely parthenogenetic reproduction in the second. Females of the third lineage reproduced either mostly via sex or mostly via parthenogenesis. Facultative parthenogenesis could be a transient state in the transition from sex to largely obligate parthenogenesis in Timema, which may have led to the repeated evolution of all-female species in the genus. It is unclear whether the populations we discovered have re-evolved sex, for example through the spontaneous production of males via X-chromosome aneuploidies, or whether they are relictual sexual populations in a species where all other lineages have evolved obligate parthenogenesis already.