New insights into the karyotype evolution of the free-living flatworm Macrostomum lignano (Platyhelminthes, Turbellaria)

Abstract

The free-living flatworm Macrostomum lignano is a model organism for evolutionary and developmental biology studies. Recently, an unusual karyotypic diversity was revealed in this species. Specifically, worms are either ‘normal’ 2n = 8, or they are aneuploid with one or two additional large chromosome(s) (i.e. 2n = 9 or 2n = 10, respectively). Aneuploid worms did not show visible behavioral or morphological abnormalities and were successful in reproduction. In this study, we generated microdissected DNA probes from chromosome 1 (further called MLI1), chromosome 2 (MLI2), and a pair of similar-sized smaller chromosomes (MLI3, MLI4). FISH using these probes revealed that MLI1 consists of contiguous regions homologous to MLI2-MLI4, suggesting that MLI1 arose due to the whole genome duplication and subsequent fusion of one full chromosome set into one large metacentric chromosome. Therefore, one presumably full haploid genome was packed into MLI1, leading to hidden tetraploidy in the M. lignano genome. The study of Macrostomum sp. 8 — a sibling species of M. lignano — revealed that it usually has one additional pair of large chromosomes (2n = 10) showing a high homology to MLI1, thus suggesting hidden hexaploidy in its genome. Possible evolutionary scenarios for the emergence of the M. lignano and Macrostomum sp. 8 genomes are discussed.

A phylogenetically informed search for an alternative Macrostomum model species, with notes on taxonomy, mating behavior, karyology, and genome size

“…The flow cytometric genome size estimates of M. janickei showed a complex pattern that approximately matched our karyotype results (Figure ), with the most prominent peak having a relative fluorescence that was on average 4.55× larger than that of D. melanogaster , which would therefore correspond to a haploid genome size in M. janickei of 779 Mbp. However, note that these large chromosomes appear to represent near identical copies (Zadesenets, Schärer, et al, ; Zadesenets et al, ), so that the actual amount of unique genome sequence might be lower. More detailed analyses of the genome organization of M. janickei are underway (K. S. Zadesenets, I. E. Jetybayev, L. Schärer, & N. B. Rubtsov, unpublished data).…”

“…These peaks have relative fluorescence values that, respectively, average 3.07x, 3.89x, and 4.69× that of D. melanogaster , so that the 2n = 8, 2n = 9, and 2n = 10 karyotypes, corresponding to haploid genome sizes of 525, 666, and 803 Mbp, respectively. However, note that the large chromosomes appear to represent near identical copies (Zadesenets, Schärer, et al, ; Zadesenets et al, ), so that the actual amount of unique genome sequence might correspond that observed in the 2n = 8 karyotype.…”

“…First, we show that M. janickei is the closest known relative of M. lignano , with which it shares an unusual hidden polyploidy. This karyotype organization is likely the result of a whole‐genome duplication event via auto‐polyploidization, leading to a fusion chromosome that varies considerably in copy number, both within and between these two species (Zadesenets, Ershov, et al, ; Zadesenets, Schärer, et al, ; Zadesenets et al, ). This species pair therefore permits to study the evolutionary outcomes of early rediploidization processes following a whole‐genome duplication (K. S. Zadesenets, I. E. Jetybayev, L. Schärer, & N. B. Rubtsov, unpublished data), shedding light on processes of genome evolution.…”

Germline-Restricted Chromosome (GRC) is Widespread among Songbirds

“…The cell suspension was dropped onto clean cold wet cover slips (60 mm x 24 mm, 0.17 mm thick), dried, and stained with 0.1% Giemsa solution (Sigma) for 3-5 min at room temperature. GRCs were identified as positive round bodies located near the spermatocytes I. Microdissection of GRC and amplification of DNA isolated from this chromosome by GenomePlex Single Cell Whole Genome Amplification Kit (WGA4) (Sigma-Aldrich) 28 .…”

Variation in sex allocation plasticity in three closely related flatworm species

“…The three study species used here are all recently described freeliving flatworms of the genus Macrostomum (Macrostomorpha, Platyhelminthes), and they include M. janickei (see also Zadesenets, Schärer, & Rubtsov, 2017;Zadesenets et al, 2016), M. cliftonensis, and M. mirumnovem (Schärer et al, 2019). Briefly, the worms used in the experiments were from laboratory cultures that were established using individuals collected from Palavas-les-Flots, near Montpellier, France, for M. janickei; from Lake Clifton, South of Perth, Western Australia, for M. cliftonensis; and from Port Phillip Bay, Queenscliff, Victoria, Australia, for M. mirumnovem (Schärer et al, 2019).…”