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BAG. Journal of basic and applied genetics

On-line version ISSN 1852-6233

BAG, J. basic appl. genet. vol.18 no.2 Ciudad Autónoma de Buenos Aires Jan./Dec. 2007



Cytogenetic characterization of seven South American species of nereididae (annelida: polychaeta): implications for the karyotypic evolution

María Claudia Ipucha1*, Cinthya Gomes Santos2, Paulo Da Cunha Lana2 and Ives José Sbalqueiro1.

1 Laboratório de Citogenética Animal, Universidade Federal do Paraná, Curitiba, PR, Brazil.
2 Centro de Estudos do Mar, Universidade Federal do Paraná, Pontal do Paraná, PR, Brazil.

Corresponding author: María Claudia Ipucha, Estafeta Postal, Playa Chapadmalal, CP 7609, Mar del Plata.
* e-mail:


This paper describes, for the first time, the cytogenetic characteristics of some South Atlantic nereidid polychaetes, collected in Paranaguá Bay, southeast Brazil. Mitotic chromosomes were obtained from regenerative tissue after amputation of the posterior tips of the animals. Seven species, of five genera, were analyzed: 1) Platynereis dumerilii (2n=28; FN=56), which is considered cosmopolitan but is suspected to be a species-complex. Karyotypic divergences were detected when comparing chromosome morphology and NOR distribution from Paraná and North American populations (2n=28; FN=56), 2) Perinereis vancaurica (2n=28; FN=56), P. anderssoni (2n=28; FN=56) and P. ponteni (2n=28; FN=54). Karyotypes and NOR distribution allowed us to relate P. anderssoni (Paraná, Brazil) to P. macropus (Italy), the former showing a derived karyotype. The remaining karyotypes of species of Perinereis showed independent divergences; 3) Pseudonereis palpata (2n=28; FN=56), known from the south and south-east coast of Brazil; 4) Nereis oligohalina (2n=28; FN=50), occurring in the South Atlantic. The last two species share the conservative chromosome number of the family; and 5) Laeonereis culveri, occurring in both the South and North Atlantic, has the highest chromosome number in the family (2n=38). We discuss the important role of karyotypic evolution in the origin of modern families of polychaetes, when the current chromosomal diversity (2n=6 to 2n=64) was developed, followed by a period of chromosomal stability of nereidid polychaetes during the dispersion which followed the Pangea breakup, with the predominance of divergences of neutral chromosomal changes, involving mainly pericentric inversions and NOR amplifications.


Se describen por primera vez, las características citogenéticas de poliquetos nereidídeos provenientes de la Bahía de Paranaguá, sudeste de Brazil. Cromosomas mitóticos fueron obtenidos a partir de tejido regenerativo después de amputar la región posterior. Fueron analizadas siete especies pertenecientes a cinco géneros: 1) Platynereis dumerilii (2n=28; FN=56), considerada cosmopolita pero que probablemente constituya un complejo de especies. Fueron detectadas divergencias cariotípicas al comparar morfología cromosómica y distribución de NORs entre poblaciones de Paraná y Atlántico Norte, 2) Perinereis vancaurica (2n=28; FN=56), P. anderssoni (2n=28; FN=56) y P. ponteni (2n=28; FN=54). Los cariotipos y distribución de NOR permitieron relacionar P. anderssoni (Paraná, Brazil) con P. macropus (Italia), presentando la primera un cariotipo derivado. Los restantes cariotipos del género mostraron divergencias independientes; 3) Pseudonereis palpata (2n=28; FN=56), distibuída en el sur y sudeste de la costa brasilera; 4) Nereis oligohalina (2n=28; FN=50), que ocurre en el Atlántico sur. Las últimas dos especies comparten el número cromosómico conservado de la familia, y 5) Laeonereis culveri, que ocurre en toda la costa Atlántica, mostró el número cromosómico más alto de la familia (2n=38). Se discute la evolución cariotípica como rol importante durante el origen de las familias modernas de Poliquetos, cuando surge la actual diversidad cromosómica (2n=6 a 2n=64), seguido de un período de establidad cariotípica dentro de la familia Nereididae durante su dispersión, posterior a la ruptura de la Pangea, a partir de la cual pasan a predominar divergencias cromosómicas neutras, involucrando principalmente inversiones pericéntricas y amplificaciones de NORs.


Polychaetes include approximately 9000 species, with thousands of names in synonymy and a quite unstable systematics (Rouse and Pleijel, 2001). About 800 species have been described for the Brazilian coast (Amaral and Jablonski, 2005). The high diversity of this group has led to an increasing need for information about the karyotypes, in order to study their evolutionary relationships, cytotaxonomy and the potential effects of environmental mutagens and other pollutants (Jha et al., 1995). Christensen (1980) stated that the cytogenetics of less than 3% of polychaete species had been investigated. More than 25 years later, the number of these investigations continues to be small.
Polychaetes show wide karyotypic variation, with diploid numbers ranging from 2n=6 in Ophryotrocha labronica, O. costlowi, O. notoglandulata and O. pacifica (Vitturi et al., 2000) to 2n=64 in Marphysa sanguinea (Hayashi et al., 1996), with a modal number of 2n=28.
Chromosome numbers in polychaetes were believed to show less variation than in other marine invertebrates (Christensen, 1980). However, as more species are being studied, the range of known diploid numbers has increased.
Sex chromosomes have been described for a limited number of species like Neanthes japonica (Sato and Ikeda, 1992) and Polydora curiosa (Korablev et al., 1999) both with XX-XY sex chromosome systems, and Dinophilus gyrociliatus (Martin and Traut, 1987) with a probable XO-XX system.
According to Seuárez (1987), karyotypic similarities revealed by Giemsa staining indicate little about genetic homologies. This is particularly true for modern taxa, where chromosome compartmentalization is reflected in G-bands and heterochromatin is present in all chromosomes, two characteristics that, although they do not cause it, can facilitate negatively heterotic chromosome rearrangement.
The G-banding technique was successfully applied in the polychaete Ophryotrocha diadema (Di and Knowles, 1992). Even though the banding observed is weak and restricted to the centromeric region, the fact of its presence contradicts the belief that late-replicating or Gbands first appeared in bony fishes (Holmquist, 1989). The small amount of heterochromatin revealed by C-bands in Platynereis dumerilii (Jha et al., 1995) is a characteristic shared by species of the genus Ophryotrocha (Vitturi et al., 2000). Taking into account the position effect of heterochromatin on gene expression, certain genes may reduce their expression when brought near a break in heterochromatin by chromosomal rearrangement (Pardue and Henning, 1990). If heterochromatin is in fact scarce in polychaetes, then practically all of the DNA would be active, even acting as supergenes. As a consequence, structural chromosome mutations would be more deleterious, and therefore a slow karyotypic evolution in this group of organisms is to be expected, as was also discussed by Vitturi et al. (2000).
Compared to other taxa, cytogenetic studies in polychaetes are limited. The scarcity of heterochromatin and G-bands does not make them strong cytogenetic characters to be used as markers for phylogenetic analysis. Therefore, karyotypes and NOR patterns are the more informative cytogenetic characters likely to reflect the course of karyotypic evolution in this group of marine invertebrates.
About 46 species of the family Nereididae have been described for the Brazilian coast (Santos and Lana, 2001) and 14 for Paranaguá Bay (Lana, 1984). Recognized as a monophyletic family (Glasby, 1993; Rouse and Fauchald, 1997), the Nereididae is one of the best-known and widespread groups of polychaetes. Only eight species of nereidids have been cytogenetically analyzed, all of them from the Northern Hemisphere (Ometz, 1963; Pesch et al., 1988; Weinberg et al., 1990; Zheng et al., 1992; Sato and Ikeda, 1992; Lipari and Vitturi, 1994 and Jha et al., 1995); most of them show a 2n=28. The karyotypes described indicate that some members of the family Nereididae had diverged karyotypically without evident morphological differentiation (Pesch et al., 1988), reinforcing the need for further cytogenetic studies as supplements to taxonomic studies.

Materials and Methods

Source of animals and culture method
Perinereis ponteni, P. anderssoni, Pseudonereis palpata and Platynereis dumerilii were collected from the rocky shores of Ilha do Mel, Paraná, Brazil (S25º33'48.6" W48º19'05.2" and S25º34'23.7" W48º18'35.4"). Perinereis vancaurica, Laeonereis acuta and Nereis oligohalina were collected from the mangrove of Perequê Channel, Pontal do Sul, Paraná, Brazil (S24º 34'25.7" W48º 34'23.7"), as listed in Table I.

Table I. Species of nereidid polychaetes studied, locality, number of specimens (N), and distribution

Adult animals were kept in aquaria supplied with natural seawater and maintained at 18±1ºC. The worms were fed a diet of chopped spinach and ground Artemia salina. The aquaria were under a photoperiod regime of 16 h of light, followed by 8 h of darkness.

Preparation of metaphase spreads and NOR banding
Mitotic cells for chromosome preparations were obtained from regenerating tips. The posterior end of each worm was excised. Approximately 2 weeks later, the regenerating tail had developed to a length of 1 to 3 mm. Metaphase chromosomes were obtained following the technique of Jha et al. (1995), with some modifications adapted to regenerating tissue. Each regenerated specimen was treated with colchicine in sea-water (at 18±1ºC) for 45 min to 1 h, following which the regenerating tips were excised and subjected to a series of 10-min hypotonic treatments (seawater : 0.075M KCl in the ratios 2:1, 1:1, 1:2 and 1:3). The tissue was then prefixed in methanol : glacial acetic acid, 3:1, for 20 min at 4ºC and fixed in four changes of fixative for 15 min each. Four to six drops of 60% glacial acetic acid were added to the tissue in a well-slide. After 5 min or until the tissue appeared translucent, the material was teased apart, first with needles and then with Pasteur pipettes. Drops of suspended material were applied to clean, hot (60ºC) slides. Then, the slides were air-dried and stained in freshly prepared 10% Giemsa solution (in phosphate buffer, pH=6.8) for 10 min, rinsed in distilled water, air-dried and examined under a light microscope.
For NOR staining, previously analyzed slides were washed in two changes of fixative for 3 min each or until all the Giemsa was eliminated. To stain nucleolar organizer regions, the protocol of Howell and Black (1980) was applied.
Chromosome morphology and the centromeric index were determined according to Levan (1964). The idiogram of each species was constructed from the centromeric index and relative length values.
Species were determined according to Lana (1984), and the voucher specimens were deposited in the collection of the Laboratório de Bentos, Centro de Estudos do Mar, Universidade Federal do Paraná, Brazil.


The chromosome number of the species analyzed was 2n=28 and FNs from 50 to 56, except for Laeonereis culveri which showed 38 chromosomes.

Platynereis dumerilii
Platynereis dumerilii showed a diploid number of 2n=28, FN=56, organized in 11 metacentric and 3 submetacentric chromosome pairs (Fig. 1a) (Table II). Nucleolar organizer regions were located in an interstitial position on the long arm of pair 1, and the terminal position on the short arm of pair 7 (Fig 1b).


Figure 1. a, karyotype of Platynereis dumerilii; b, idiogram of Platynereis dumerilii showing the chromosome morphology and position of NORs (dark regions); c, karyotype of Perinereis ponteni; d, idiogram of Perinereis ponteni showing the chromosome morphology and position of NOR (dark region); e, karyotype of Perinereis anderssoni; f, idiogram of Perinereis anderssoni showing the chromosome morphology and NOR position (dark regions). Scale bar: 10 μm.

Table II. Relative length (RL) and centromeric index (CI) of the chromosomes (CNº) of six species of Nereididae from southeast Brazil. In parentheses, chromosome morphology according to the nomenclature of Levan et al. (1964): m = metacentric; sm = submetacentric; st = subtelocentric; t (T) = telocentric.

Perinereis ponteni
The karyotype of Perinereis ponteni showed a diploid number of 2n=28, FN=54, organized in 13 metacentric and 1 telocentric pairs (Fig. 1c) (Table II). The NOR region was evidenced in the telomeric position on the long arm of pair 1 (Fig. 1d).

Perinereis anderssoni
Perinereis anderssoni showed a 2n=28, FN=56. The 14 biarmed chromosomes were grouped in 10 metacentric and 4 submetacentric pairs (Fig. 1e) (Table II). Silver staining showed a variation from 2 to 10 NORs, both inter- and intraindividual.
Figure 1f, showing the NOR position, represents the maximum of NORs detected with the following distribution: telomeric region on the long arm of pair 7, telomeric position on the short arm of pair 8, telomeric position and interstitial on the short and long arm of pair 11, respectively; and telomeric region on the short arm of pair 12.

Perinereis vancaurica
Perinereis vancaurica showed a diploid number 2n=28 and FN=56, organized in 13 metacentric and 1 submetacentric pairs (Fig. 2a) (Table II). Silver stained NORs were located in a telomeric position on the long arm of pair 2, and the telomeric region on the short arm of pair 9 (Fig. 2b).

Figure 2. a, karyotype of Perinereis vancaurica; b, idiogram of Perinereis vancaurica showing the chromosome morphology and NORs position (dark regions); c, karyotype of Nereis oligohalina; d, idiogram of Nereis oligohalina showing the chromosome morphology and NORs position; e, karyotype of Pseudonereis palpata; f, idiogram of Pseudonereis palpata showing the chromosome morphology and NORs position. Scale bar: 10 μm

Pseudonereis palpata
Pseudonereis palpata showed a diploid number of 2n=28, FN=56, organized in 11 metacentric and 3 submetacentric pairs (Fig. 2e) (Table II). NOR regions were observed in a telomeric position on the short arm of pair 13 (Fig. 2f). Intra-individual variations in the number of arms marked were observed.

Nereis oligohalina
Nereis oligohalina showed a diploid number of 2n=28, FN=50 organized in 7 metacentric, 1 submetacentric, 3 subtelocentric and 3 telocentric pairs (Fig. 2c) (Table II). NORs were evidenced in a telomeric position on the long arm of pair 1 and the short arm of pair 12 (Fig. 2d).

Laeonereis culveri
Laeonereis culveri showed a diploid number of 2n=38. It was not possible to determine with precision the FN, because of the poor chromosome quality of the two metaphases analyzed. For the same reason it was impossible to construct its idiogram.
None of the cells stained with silver showed a clear identification of the NOR position.


Table III shows the current karyotypic information on members of Nereididae.
Chromosome size and diploid number within polychaetes tend to be lower and less variable compared with other marine invertebrates such as oligochaetes (2n=22-190, modal number=36) (Gregory and Hebert, 2002). The diploid chromosome number of 2n=28 in six of the seven nereidid species that we analyzed is the same as reported for most other nereidids (Jha et al., 1995; Lipari and Vitturi, 1994; Zheng et al., 1992; Sato and Ikeda, 1992). The exceptions to this diploid number are represented by Neanthes arenaceodentata (2n=18-24) (Pesch, 1988); Perinereis cultrifera (2n=34) and Hediste diversicolor (2n=32) (Ometz, 1963); and Laeonereis culveri (2n=36) (present report).

Table III. Cytogenetic data for species of Nereididae. Locality (IM = Ilha do Mel, NA = North Atlantic, PC = Perequê Channel, IT = Italy, PA = Pas-de-Calais, NP = North Pacific, SC = Scandinavia, OR = Omoi River, Japan); 2n = diploid number; FN = fundamental number; No NOR = highest number of NOR marks; int = interstitial; ter = terminal location (telomeric); p = short arm; q = long arm; N/d = Not determined; ? = number not determined.

Variation in the diploid chromosome number in polychaetes is sometimes related to their reproductive pattern, such as in species of Dorvilleidae (Åkesson, 1975); the nereid N. arenaceodentata (2n=18-22), where the young receive paternal care within the tube; and L. culveri (2n=38), which like most nereidids is gonochoristic, but has atokous reproductors like many brackish-water nereidids (Klesch, 1970)
All other analyzed nereidid species with 2n=28, reproduce by a process of epigamy. While variation on chromosome number within a taxon is unlikely to be linked to a single cause or effect (King, 1993), the conservative nature of the rest of the group implies that interspecific chromosome variation may be related in some way to the reproductive pattern, even though their mechanisms are still unclear.

Platynereis dumerilii

Platynereis dumerilii is a cosmopolitan species, suspected to be a species-complex (Fauchald, 1977). When cytogenetic data of P. dumerilii from Ilha do Mel (=IM) was compared to that reported from the North Atlantic (=NA) by Jha et al. (1995), similarities and differences were found. Individuals from both localities showed a 2n=28, FN=56 with the same chromosome relative length. Karyotypic classification differed in the number of metacentric and submetacentric chromosomes, suggesting that pericentric inversions are involved in the diverging of the two karyotypes.
This affirmation is based on comparative analysis between the idiograms and relative length from both populations: NA (Jha et al., 1995) and IM (present report). This allows for comparisons between each chromosome pair. By regrouping both karyotypes by the relative length (independent from chromosome morphology), NORs from NA are located on chromosome pairs 9 and 11, whereas NORs from IM are located on chromosomes 1 (interstitial band) and 9.
In respect to the phylogenetic relationships among closely related species, the presence of a single pair of NOR bearing chromosomes is considered ancestral in most vertebrates (Schmid, 1978; Amemiya and Gold, 1990), some invertebrates (Vitturi et al., 1991), and some polychaetes (Sella et al., 1995). All other NOR phenotypes are considered evolved (Hsu et al., 1975). The NOR cytotypes found in both populations of Platynereis dumerilii led to the hypotheses of an ancestral karyotype of 2n=28 with one NOR located on chromosome 9. During or after the radiation of the species, NORs would be independently amplified to other chromosomes, giving the two divergent NOR cytotypes, each one occupying its respective geographic distribution. P. dumerilii (IM) is the first polychaete species known to show a NOR in an interstitial position.
The differences between populations of P. dumerilii found here do not support the idea of a species-complex, but rather a polytypic species.
Karyotypic changes such as NOR amplification are considered neutral chromosomal mutations (King, 1993); although they can follow the adaptive radiation of populations, they do not have the capacity to cause evolutionary divergence between species. If such a species-complex exists, as we also believe, specific differences are not reflected cytogenetically, or at least not between populations from the same vertical range.


Perinereis is certainly polyphyletic, and is one of the most polytypic genera among nereidids. The three species analyzed: P. ponteni, P. anderssoni and P. vancaurica, together with P. macropus (Lipari and Vitturi, 1994) and P. nuntia (Zheng et al., 1992) show a 2n=28 with some differences in their karyotypic morphology. They all have a FN=56, except for P. ponteni with a FN=54 due to a small telocentric pair (Fig. 1c-d, Table III), which could have emerged by a pericentric inversion.
Perinereis species show a tendency toward karyotypic stability. The differences in chromosomal morphology, taking into account the general scarcity of heterochromatin in polychaetes, led us to suppose that pericentric inversions were one of the main mechanisms responsible for the diversification of karyotypes.
An exception to karyotype stability was observed in P. cultrifera (2n=34). Bakken and Wilson (2005) established four main clades of nereidids with paragnaths, separating P. cultrifera from the group of species with 2n=28. Cytogenetic results reaffirm the possibility of more than one karyotypic tendency within the genus Perinereis.
The NORs patterns showed wide variation between species of Perinereis: one to four pairs of bearing chromosomes. The karyotype of P. ponteni (with a single NOR pair) could be considered ancestral, while P. macropus, P. vancaurica and P. anderssoni (with more than one pair) derived cytotypes, although not necessarily as a group of species with any shared characteristic. In fact, the NOR cytotypes of the last three species are quite diverse. Seemingly, P. macropus, with a small metacentric bearing chromosome, appears to be ancestral to P. anderssoni, with two small metacentric of NOR bearing chromosomes. The close chromosomal relationship between P. macropus and P. anderssoni is in agreement with the morphological affinities proposed by Hutchings et al. (1991). On the other hand, the patterns of NOR localization in P. ponteni and P. vancaurica indicate independent origins in respect to the former species and also independent between them. However, this recognized polyphyletic genus requires further investigation to better clarify its karyotypic relationships.

Pseudonereis palpata

Pseudonereis palpata also showed the most common diploid number among nereidids (2n=28) and a karyotype similar to that of the species of Perinereis, characterized by the presence of the three first chromosome pairs relatively larger compared to the rest of the complement. The presence of a single pair of NOR bearing chromosomes indicates a conservation of an ancestral condition.


Nereis is the most karyotypically diversified genus among nereidids. The names Nereis acuminata, Neanthes arenaceodentata and Neanthes caudata have been indiscriminately applied to widely distributed nereidid populations from the North Atlantic in Europe as well the North American Atlantic and Pacific coasts. It is not the aim of the present study to clarify the confusing taxonomic history of this taxon, this is considered cosmopolitan based on the morphological similarities of geographically isolated species. Despite this, it is almost certain that under these names, we have a complex of morphologically very closely related species. Pesch et al. (1988) analyzed two populations, referred to as Neanthes (Nereis) arenaceodentata, from the Atlantic (Connecticut) and the Pacific (California) coast respectively. The karyotypic differences found (2n=18, FN=34 and 2n=22, FN=24) led the authors to recognize that these populations represent different species. Subsequently, Weinberg et al. (1990) also analyzed two allopatric populations of Nereis acuminata from California and Connecticut, but from different localities from Pesch´s. Once again, the extreme dissimilarity in karyotypes found: 2n=18, FN=36 (Pacific coast) and 2n=22, FN=22 (Atlantic coast), and the reproductive isolation observed in the laboratory, led the authors to propose that these populations belong to different species.
Thus, the cytogenetic and behavioral data presented by Pesch et al. (1988) and Weinberg et al. (1990) suggest that the species occurring on the Atlantic coast should be recognized as different from the one on the Pacific coast. Additionally, both would be polytypic species, due to the existence of populations with different FN in both cases. It is quite probable that similar situations occur in other nereidid taxa that are presently thought to be cosmopolitan, where true speciescomplexes may be revealed as more ecological, cytological, and molecular studies are intensified.
Nereis oligohalina shares the 2n=28 of the majority of nereidids. However, when compared to other species of the genus, such as N. acuminata and N. arenaceodentata, it does not show a close relation from the cytogenetic point of view. Fauchald (1977) stated that Nereis as a genus constitutes an heterogeneous group of species, which could be divided into smaller, more or less homogeneous groups; but such a revision still needs to be done.

Laeonereis culveri

Laeonereis culveri was the only species with a diploid number superior to the characteristic 2n=28, and also has the highest number so far recorded for a nereidid: 2n=38. Another distinctive characteristic of the species was the presence of a very high number of nucleoli (revealed by silver impregnation), about 20 per interphasic cell. In all the other species studied here, the number of interphasic nucleoli was consistent with the quantity of NOR bearing chromosomes, attaining a maximum of five per cell. Although chromosome banding by silver impregnation was not successful in L. culveri, the quantity of nucleoli detected led us to suppose the existence of numerous NORbearing chromosomes, suggesting an elevated capacity of DNAr amplification and dispersion throughout non-homologous chromosomes. Santos et al (2005) and Rouse and Pleijel (2001) put L. culveri in a separate clade from the other species analyzed herein. The cytogenetic particularities of L. culveri support once again the idea that it followed an independent course of evolution.


The karyotypes determined in Perinereis ponteni (2n=28; FN=54), P. anderssoni (2n=28; FN=56), P. vancaurica (2n=28; FN=56), Pseudonereis palpata (2n=28; FN=56), Nereis oligohalina (2n=28; FN=56) and Platynereis dumerilii (2n=28; FN=56), reaffirm the karyotypic stability of nereidid species, as previously suggested by Jha et al. (1995). The conserved diploid and fundamental numbers on one hand, and variation in the karyotypic formulae on the other, indicate that pericentric inversions apparently represent the main mechanism responsible for chromosomal evolution in the family Nereididae.
Members of all the polychaete families, including most of their genera, have been reported in all oceans at all depths (Glasby and Alvarez, 1999). According to Fauchald (1984), the worldwide distribution of families and genera is seen as a consequence of the long evolutionary history of the group. The limited fossil records showed that recent families differentiated well before the Pangea breakup, in contrast to teleosts, echinoderms and crustaceans, which experienced later radiations.
Pseudonereis palpata (2n=28), one of the karyotypically uniform species of nereidids, is endemic to the South Atlantic Ocean, suggesting that the chromosomal stability of the group was reached before the members of the family colonized the present South Atlantic coast.
Karyotypic stability in nereidids has been attained at the family level. The current cytogenetic knowledge of polychaetes supports the refutable hypothesis that the karyotypic evolution of this group has passed through at least two distinct periods: 1. A period of marked chromosomal evolution during the origin and establishment of the modern families, before the Pangea breakup and revealed by the current diversity shown by the higher categories, with 2n=6 to 2n=64. It occurred at a restricted period of time (not necessarily short) of adaptive radiation of the group, interrupting its action during the later period of dispersion, which propitiated. 2. A period characterized by karyotypic stability in nereidids extended until the present and entailed morphological diversification without negatively heterotic chromosomal changes, involving mainly pericentric inversions in some families and centric fusion in others. A similar pattern has been observed in families of turtles (Bull et al., 1974; Bickham and Baker, 1976).
Modern nereidid species present one of the most favorable characteristics for karyotypic conservation: small genomes and an almost complete lack of heterochromatic (or neutral) regions, a characteristic shared by members of the class Hirudinea (Vitturi et al., 2002). Nevertheless, characteristics of the structure of the genome and the potential for transposable elements to cause consistent breaks at particular sites, can lead to the formation of certain types of rearrangements involving specific chromosomes and in some cases the whole genome (King, 1993). The apparent chromosomal uniformity displayed by entire families of plants and animals (King, 1993) suggests that particular genomes have a high potential for change, whereas others remain frozen in time. The type and frequency of rearrangements depend on the DNA organization. If, as observed by Collins and Rubin (1984), the insertion sites of mobile elements are not random, then the site of chromosomal rearrangements and their frequency are also non-random. From the present knowledge about the cytogenetics of Nereididae we have no doubt about the existence of a constancy in diploid number, at the same time that chromosomal morphology attained a certain level of variation. The only type of chromosomal rearrangement able to alter the centromere position without modifying the 2n and FN is, apparently, pericentric inversions. This is probably the main mechanism of chromosomal evolution operating in the modern species and genera of the family.


We thank Verônica Maria de Oliveira and André Senna Garraffoni for their help and support throughout the work, and professor Iglenir Cavalli for his assistance and comments on a previous version of the manuscript. We are also grateful to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior) for financial support.


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