Abstract
Parthenogenesis has evolved independently in more than 10 Drosophila species. Most cases are tychoparthenogenesis, which is occasional or accidental parthenogenesis in normally bisexual species with a low hatching rate of eggs produced by virgin females; this form is presumed to be an early stage of parthenogenesis. To address how parthenogenesis and sexual reproduction coexist in Drosophila populations, we investigated several reproductive traits, including the fertility, parthenogenetic capability, diploidization mechanisms, and mating propensity of parthenogenetic D. albomicans. The fertility of mated parthenogenetic females was significantly higher than that of virgin females. The mated females could still produce parthenogenetic offspring but predominantly produced offspring by sexual reproduction. Both mated parthenogenetic females and their parthenogenetic-sexual descendants were capable of parthenogenesis. The alleles responsible for parthenogenesis can be propagated through both parthenogenesis and sexual reproduction. As diploidy is restored predominantly by gamete duplication, heterozygosity would be very low in parthenogenetic individuals. Hence, genetic variation in parthenogenetic genomes would result from sexual reproduction. The mating propensity of females after more than 20 years of isolation from males was decreased. If mutations reducing mating propensities could occur under male-limited conditions in natural populations, decreased mating propensity might accelerate tychoparthenogenesis through a positive feedback mechanism. This process provides an opportunity for the evolution of obligate parthenogenesis. Therefore, the persistence of facultative parthenogenesis may be an adaptive reproductive strategy in Drosophila when a few founders colonize a new niche or when small populations are distributed at the edge of a species' range, consistent with models of geographical parthenogenesis.
Highlights
Parthenogenesis typically involves females that lay unfertilized eggs that develop into individuals and has independently and recurrently evolved from sexually reproducing ancestors in many multicellular organisms [1, 2]
Diploidization usually occurs through three main cytological mechanisms: gamete duplication, in which chromosome doubling occurs after meiosis II; the central fusion of two nuclei derived from different meiosis I nuclei; and the terminal fusion of two nuclei derived from the same meiosis I nucleus [3,4,5,6,7]
As in other parthenogenetic Drosophila species [21], long-term (20 years,370 generations) isolation from males resulted in a reduced mating propensity in parthenogenetic D. albomicans females
Summary
Parthenogenesis typically involves females that lay unfertilized eggs that develop into individuals and has independently and recurrently evolved from sexually reproducing ancestors in many multicellular organisms [1, 2]. Most parthenogenetic lineages are facultative, i.e., individuals can reproduce by both parthenogenesis and sexual reproduction [3,4,5,6,7], whereas very few lineages, such as bdelloid rotifers, have been successful for millions of years under obligate parthenogenesis, in which individuals are exclusively parthenogenetic [8] In the latter case, alternate genetic mechanisms that facilitate long-term persistence involving DNA damage and repair are involved [9]. If gamete duplication is the major mechanism, such as in the wellstudied facultatively parthenogenetic cases of D. mercatorum [13, 14, 16, 24] and D. ananassae complex species [15, 25, 36], the genetic variation in parthenogenetic individuals would be very low These four reproductive features might explain why many Drosophila parthenogenetic lineages are facultative rather than obligate
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