Hypodermic self-insemination as a reproductive assurance strategy

Self-fertilization occurs in a broad range of hermaphroditic plants and animals, and is often thought to evolve as a reproductive assurance strategy under ecological conditions that disfavour or prevent outcrossing. Nevertheless, selfing ability is far from ubiquitous among hermaphrodites, and may be constrained in taxa where the male and female gametes of the same individual cannot easily meet. Here, we report an extraordinary selfing mechanism in one such species, the free-living flatworm Macrostomum hystrix. To test the hypothesis that adaptations to hypodermic insemination of the mating partner under outcrossing also facilitate selfing, we experimentally manipulated the social environment of these transparent flatworms and then observed the spatial distribution of received sperm in vivo . We find that this distribution differs radically between conditions allowing or preventing outcrossing, implying that isolated individuals use their needle-like stylet (male copulatory organ) to inject own sperm into their anterior body region, including into their own head, from where they then apparently migrate to the site of (self-)fertilization. Conferring the ability to self could thus be an additional consequence of hypodermic insemination, a widespread fertilization mode that is especially prevalent among simultaneously hermaphroditic animals and probably evolves due to sexual conflict over the transfer and subsequent fate of sperm.

Reproductive success of Acacia longifolia (Fabaceae, Mimosoideae) in native and invasive populations

The reproductive biology of exotic species affects their capacity to become naturalised and invasive in non-native areas. Selfing is a common trait in many invasive plants probably because it provides reproductive assurance under low availability of pollination vectors and sexual partners. Nonetheless, the predominantly self-incompatible Australian Acacia species are among the most aggressive plants worldwide. To address whether there have been changes in selfing ability and natural reproductive success of A. longifolia during invasion, we compared one population in the invaded area (Portugal) with one population in the native range (Australia). We specifically assessed floral traits, fruit set and offspring traits for selfing and open-pollination treatments. Within each pollination treatment, no differences were found between areas, suggesting that the level of self-compatibility has not changed during invasion. However, the number of aborted seeds and seed size were significantly different between pollination treatments in Australia but not in Portugal. There were significant differences in the number of seeds per pod and in seed weight between ranges. A lower number of aborted seeds, a higher number of fully developed seeds and a greater seed size were found in the invaded area for both pollination treatments. In spite of the low selfing ability of A. longifolia in the invaded area, there was an increase in the quantity and size of the seeds produced in the new region, even for self-pollinated fruits, which might contribute to A. longifolia invasiveness.

Low reproductive success of hay-scented fern (Dennstaedtia punctilobula) regardless of inbreeding level or time since disturbance

Self-fertilization can facilitate the colonization of new habitats because it allows a single individual to found a population. Here we investigated the relationship between mating systems and colonization in hay-scented fern (Dennstaedtia punctilobula (Michx.) T.Moore). Throughout eastern North America, this species has been called a “native invasive” for its tendency to dominate forest understories disturbed by logging, inhibiting tree regeneration. Thus, it is important to understand the mechanisms of its spread. We hypothesized that if populations were founded through selfing, then populations disturbed more recently would retain higher selfing ability; this pattern would demonstrate an important link between mating systems and colonization. For four populations logged at different times in the past, we compared the sporophyte production of gametophytes at different levels of inbreeding (intragametophytic selfing, intergametophytic selfing, and outcrossing) using laboratory crosses. Across all treatments, only 9.8% of gametophytes formed sporophytes (N = 400 gametophytes). Neither inbreeding level nor time since disturbance affected sporophyte production. Selfing ability did not differ across populations logged at different times; there was no interaction between inbreeding level and time since disturbance. The low reproductive success of D. punctilobula, regardless of inbreeding level or time since disturbance, suggests that population establishment and expansion via sexual reproduction may be relatively rare in this clonal species.

Floral and pollination biology of three sympatric Vaccinium (Ericaceae) species in the Upper Ardennes, Belgium

Comparative studies of the reproductive biology and pollination ecology of closely related species allow us to test several ideas related to the evolution of selfing taxa from outcrossing ancestors. The existence of closely related species in the same habitat provides a particularly useful opportunity to examine this issue. A variety of floral traits likely to be associated with the reproductive system of three sympatric Vaccinium species (V. myrtillus, V. vitis-idaea, and V. uliginosum) were quantified in a heathland in the Upper Ardennes, Belgium. These traits included the length and width of the corolla, the number and size of the anthers, the number of pollen tetrads and ovules, and the length of the style. Pollen to ovule ratios suggest a mixed mating system in the three species. The greater pollen to ovule ratio and stigma–anther separation in V. vitis-idaea suggest that it functions more as an outcrosser than the two congeners. The effects of caging, emasculation, and artificial pollination on fruit and seed set differed among years and among the three species. Supplementary pollination increased fruit set and fruit characteristics (particularly seed number) relative to natural pollination in the three species. The three species showed a varied but poor capacity to self in the absence of pollinators. Seed set per fruit was lower in the spontaneously selfed flowers in comparison with hand-crossed pollinated flowers in V. myrtillus and V. vitis-idaea but not in V. uliginosum. This higher ability to self in V. uliginosum indicates a lower capacity to self in the absence of pollinators. However, all the three species were at least partially self-compatible. Together the floral traits and selfing ability suggest that the polyploid V. uliginosum appears to be more highly selfing than the two diploids, particularly V. vitis-idaea. Keywords: floral biology, mixed mating, mating system, Vaccinium, seed set.