Occasional sexual reproduction significantly affects the population structure of the widespread, predominantly asexually reproducing marine worm Lineus sanguineus (Nemertea: Pilidiophora)

Abstract

Asexual reproduction by fissiparity has only been described from very few of the approximately 1300 nemertean species that otherwise mainly reproduce sexually. The best studied fissiparous species, Lineus sanguineus (Rathke 1799), is a cosmopolitan heteronemertean species inhabiting intertidal habitats of temperate coasts. Although sexual reproduction has never been described, molecular data suggest that sexual reproduction substantially shapes the genetic structure of the investigated populations. In an attempt to clarify the extent of sexual reproduction, three gene fragments (COI, 16S, ITS) were sequenced for 108 specimens sampled in 8 European localities. The results of a phylogenetic analysis and haplotype network showed no clear distinction between different populations, thus indicating the presence of sexual reproduction. Furthermore, we provide circumstantial evidence for the presence of a comparably long-lived planktonic larval stage as present in the closest relatives of L. sanguineus. To further understand the impact of abiotic factors on sexual reproduction and fissiparity, the effect of different temperature and illumination regimes on reproductive behavior and fragmentation was studied in specimens from a population from Bergen, Norway that share the same haplotype. Experimental setups represented summer (long light period and elevated temperature) and winter (short light and decreased temperature) conditions. Under winter conditions, a higher number of animals remained sexually mature and at least one specimen shed eggs on one occasion. Thus, although short light and/or low temperatures are most likely the influential factors on sexual maturity, the factors that influence fissiparity are less clear. The results of this study further solidify the cosmopolitan status of L. sanguineus and clarify the population structuring of this species. In addition, the study provides first data on the dynamics of sexual and asexual reproduction modes on which future investigations will have to expand, especially regarding genetic and physiological aspects.

Phylogeographic characterization of the ribbon worm Lineus sanguineus populations from the Southwestern Atlantic (Heteronemertea: Lineidae): a true case of cosmopolitanism

“…Our phylogenetic analyses evidenced this species shows no geographic segregation across different continents and distant localities. Those patterns agree with recent results from previous studies (Runnels 2013, Ament-Velasquez et al 2016, Sagorny and Döhren 2022, which also did not found relevant geographic segregation for specimens from different continents.…”

“…However, in the last decade, some studies regarding this species' population genetics with microsatellites, mtDNA, ITS and SNPs showed a lack of genetic structure within and between North American, European and Argentinian populations (Runnels 2013;Ament-Velasquez et al 2016;Sagorny and Döhren 2022), indicating sexual reproduction. In addition, Sagorny and Döhren (2022) were able to document what might be a L. sanguineus larva due to its morphological similarity to the long-lived planktonic larva from the closely related lineid species, Riseriellus occultus (Rogers, Junoy, Gibson and Thorpe, 1993), evidencing L. sanguineus dispersion capability. Lineus sanguineus can be easily found on the Brazilian coastline (Correa, 1955), but, similar to the Nemertean phylum in general, little is known about its diversity and population genetics in the Southwestern Atlantic.…”

A Procedure for Modeling Genetic Diversity Distortions in Populations of Organisms with Mixed Reproductive Strategies

“…One such metric is the frequency of recombination events, which can be used as an indicator of the relative contribution of sexual and asexual reproduction to the gene pool of the population [16,19,24,25]. For example, if recombination events are relatively rare, this suggests that asexual reproduction is dominant and genetic diversity is primarily generated through mutation [26]. Conversely, if recombination events are frequent, this suggests that sexual reproduction is dominant and genetic diversity is primarily generated through recombination.…”

“…Heterozygosity is a measure of genetic diversity that reflects the presence of different alleles at a given gene locus. A population that reproduces primarily through sexual reproduction is expected to higher levels of heterozygosity, as a result of the shuffling of genetic material during meiosis and fertilization [3,26].…”

Molecular evidence for a new endemic species of Acartia (Copepoda, Calanoida) from the Southeast Pacific coast

“…However, the use of DNA barcoding has shown that the genetic diversity of marine populations is underestimated, especially in populations that are difficult to identify and sample [65][66][67] . Although increasing the sampling effort is not the definitive solution, because ecological characteristics of the species studied, such as the reproductive strategy, mitochondrial inheritance mode or spatial and temporal distribution and abundance of haplotypes are capable of influencing diversity estimates and the required sampling effort [67][68][69][70][71] . We had the possibility of partially exploring the temporal variability of the Concepcion population, with the aim of finding previously undetected mitochondrial haplotypes, but it was interesting that although the population of Concepcion was sampled on two occasions, more than 10 years apart (see Suppl.…”