Chromosome numbers in three species groups of freshwater flatworms increase with increasing latitude

Abstract

Polyploidy in combination with parthenogenesis offers advantages for plasticity and the evolution of a broad ecological tolerance of species. Therefore, a positive correlation between the level of ploidy and increasing latitude as a surrogate for environmental harshness has been suggested. Such a positive correlation is well documented for plants, but examples for animals are still rare. Species of flatworms (Platyhelminthes) are widely distributed, show a remarkably wide range of chromosome numbers, and offer therefore good model systems to study the geographical distribution of chromosome numbers. We analyzed published data on counts of chromosome numbers and geographical information of three flatworm “species” (Phagocata vitta, Polycelis felina and Crenobia alpina) sampled across Europe (220 populations). We used the mean chromosome number across individuals of a population as a proxy for the level of ploidy within populations, and we tested for relationships of this variable with latitude, mode of reproduction (sexual, asexual or both) and environmental variables (annual mean temperature, mean diurnal temperature range, mean precipitation and net primary production). The mean chromosome numbers of all three species increased with latitude and decreased with mean annual temperature. For two species, chromosome number also decreased with mean precipitation and net primary production. Furthermore, high chromosome numbers within species were accompanied with a loss of sexual reproduction. The variation of chromosome numbers within individuals of two of the three species increased with latitude. Our results support the hypothesis that polyploid lineages are able to cope with harsh climatic conditions at high latitudes. Furthermore, we propose that asexual reproduction in populations with high levels of polyploidization stabilizes hybridization events. Chromosomal irregularities within individuals tend to become more frequent at the extreme environments of high latitudes, presumably because of mitotic errors and downsizing of the genome.

Shrinking body sizes in response to warming: explanations for the temperature–size rule with special emphasis on the role of oxygen

“…These orders exhibit a significant difference in genome size (and hence probably cell size) [ t ‐test: P = 1.005 × 10 −7 ; calculated from data in Gregory, 2018; see also Wyngaard & Rasch, 2000]. Similarly, aquatic species living in cold environments tend to have larger genomes (Dufresne & Jeffery, 2011; Lorch et al ., 2016; Alfsnes et al ., 2017; Jeffery, Yampolsky, & Gregory, 2017), suggesting a cell‐size parallel with Bergmann's rule, at least for aquatic animals. In general, animals increase body size mainly through cell proliferation during early development, but by cell growth in later life (Kammenga et al ., 2007; Czarnołęski et al ., 2008, but see Aguilar‐Alberola & Mesquita‐Joanes, 2014; Horne et al ., 2019) and this fits with the TSR being less pronounced for egg size, manifesting itself in later life stages (Forster, Hirst, & Atkinson, 2011; Forster & Hirst, 2012).…”

The first mitochondrial genomes of endosymbiotic rhabdocoels illustrate evolutionary relaxation of atp8 and genome plasticity in flatworms

“…Indeed, as more flatworm species are incorporated into molecular studies, a greater number of genomic peculiarities are being discovered. Examples include several instances of polyploidy [16], high degrees of genomic polymorphy [17], [18] and 18S rDNA polymorphisms [19]. In the case of mitochondrial (mt) genomes, this variation is well illustrated by technical difficulties in protocols that are relatively straightforward with other taxa.…”

Triploidy in zebrafish larvae: Effects on gene expression, cell size and cell number, growth, development and swimming performance

“…In studies on the ecophysiology of ectotherms, a field gaining more and more interest deals with the regulation and consequences of cell size [1]. For ectotherms, patterns in cell size across thermal clines associated with latitude and altitude have been documented, where animals are generally composed of larger cells in the cold [2][3][4]. In addition, when ambient temperatures are experimentally lowered while rearing ectotherms, cell size tends to increase and this holds for phyla as diverse as nematodes (Caenorhabditis elegans; [5]), arthropods (Daphnia magna; [6]) and chordates (the edible frog Pelophylax esculentus; [7]).…”