Evidence for Karyotype Polymorphism in the Free-Living Flatworm, Macrostomum lignano, a Model Organism for Evolutionary and Developmental Biology

Abstract

Over the past decade, the free-living flatworm Macrostomum lignano has been successfully used in many areas of biology, including embryology, stem cells, sexual selection, bioadhesion and aging. The increased use of this powerful laboratory model, including the establishment of genomic resources and tools, makes it essential to have a detailed description of the chromosome organization of this species, previously suggested to have a karyotype with 2n = 8 and one pair of large and three pairs of small metacentric chromosomes. We performed cytogenetic analyses for chromosomes of one commonly used inbred line of M. lignano (called DV1) and uncovered unexpected chromosome number variation in the form of aneuploidies of the largest chromosomes. These results prompted us to perform karyotypic studies in individual specimens of this and other lines of M. lignano reared under laboratory conditions, as well as in freshly field-collected specimens from different natural populations. Our analyses revealed a high frequency of aneuploids and in some cases other numerical and structural chromosome abnormalities in laboratory-reared lines of M. lignano, and some cases of aneuploidy were also found in freshly field-collected specimens. Moreover, karyological analyses were performed in specimens of three further species: Macrostomum sp. 8 (a close relative of M. lignano), M. spirale and M. hystrix. Macrostomum sp. 8 showed a karyotype that was similar to that of M. lignano, with tetrasomy for its largest chromosome being the most common karyotype, while the other two species showed a simpler karyotype that is more typical of the genus Macrostomum. These findings suggest that M. lignano and Macrostomum sp. 8 can be used as new models for studying processes of partial genome duplication in genome evolution.

Transcriptional signatures of somatic neoblasts and germline cells in Macrostomum lignano

“…However, in contrast to the S. mediterranea transcriptome assembly, more than half of the complete gene models are not single-copy but duplicated in the M. lignano transcriptome (Figure 1—figure supplement 2B). The presence of multiple copies of the genes that usually are single-copy in other organisms can be explained by the observation that M. lignano DV1 line used for the transcriptome assembly has a duplicated large chromosome, and hence a likely recent partial genome duplication (Zadesenets et al, 2016). …”

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

“…Subsequent karyological analyses have confirmed this unusual karyotype and also revealed that individual M. lignano can deviate from the 2n = 8 karyotype and that such variation segregates within freshly field‐collected worms, outbred laboratory cultures, and also inbred lines (Zadesenets et al, ). Most of this variation involves the presence of either 2, 3, or 4 copies of the large chromosome (yielding 2n = 8, 2n = 9, and 2n = 10 individuals, respectively), but individuals with additional karyotypes (including some with chromosomal rearrangements) also occur at low frequency (Zadesenets et al, ). More recent analyses have further shown that the large chromosome represents a fusion product of one full haploid chromosome set, making the 2n = 8, 2n = 9, and 2n = 10 karyotypes effectively hidden tetra‐, penta‐, and hexaploids, respectively.…”

“…Details on species and specimens included are given in Table . Briefly, M. lignano was collected in the Northern Adriatic Sea (including the type locality) and the Aegean Sea (Ladurner et al, ; Zadesenets et al, ), and M. janickei was collected in the Gulf of Lion (Zadesenets et al, ; called Macrostomum sp. 8 therein).…”

“…8 therein). Long‐term laboratory cultures have been established from these species, including genetically outbred cultures (of both M. lignano and M. janickei ) and inbred lines (of M. lignano ) (Vellnow, Vizoso, Viktorin, & Schärer, ; Zadesenets et al, ). Furthermore, we have collected M. cliftonensis from the shores of Lake Clifton, an increasingly hypersaline coastal lake South of Perth (Western Australia; Lane, Clarke, & Winchcombe, ), as well as M. mirumnovem from several sites in Port Philipp Bay near Queenscliff (Victoria).…”

“…Metaphase plates were prepared from individual worms of M. janickei , M. cliftonensis , M. mirumnovem , and M. hystrix , using the single‐worm karyotyping technique described previously (Zadesenets et al, ). Chromosomes were counterstained with VECTASHIELD Antifade Mounting Medium with DAPI (Vector Laboratories).…”