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Latin american journal of aquatic research

versión On-line ISSN 0718-560X

Lat. Am. J. Aquat. Res. vol.42 no.1 Valparaíso mar. 2014

http://dx.doi.org/103856/vol42-issue1-fulltext-14 

Research Article

 

Differences in sperm ultrastructure between Mytilus chilensis and Mytilus galloprovincialis (Bivalvia, Mytilidae): could be used as a taxonomic trait?

Diferencias en la ultraestructura espermática entre Mytilus chilensis y Mytilus galloprovincialis (Bivalvia, Mytilidae): ¿Se puede utilizar como un carácter taxonómico?

 

Pablo A. Oyarzún1,2, Jorge E. Toro1, Orlando Garrido1, Carolina Briones3 & Ricardo Guiñez3

1 Instituto de Ciencias Marinas y Limnológicas (ICML), Facultad de Ciencias Universidad Austral de Chile, Independencia 641, Valdivia, Chile Corresponding author: Jorge E. Toro (jtoro@uach.cl)
2 Programa de Doctorado en Biología Marina, Universidad Austral de Chile Independencia 641, Valdivia, Chile
3 Instituto de Ciencias Naturales "Alexander von Humboldt", Facultad de Ciencias del Mar y de Recursos Biológicos, Universidad de Antofagasta, Angamos 601, Antofagasta, Chile


ABSTRACT. The sperm ultrastructure has been used to solve several systematic and phylogenetic problems in marine invertebrates. The sperm ultrastructure of the Chilean mussel Mytilus chilensis and Mytilus galloprovincialis corresponds to the ect-aquasperm type. Sperm from both taxa measured 55-60 μm between head (acrosome + nucleus), midpiece (only 5 mitochondria) and the flagellum which in its end piece has a smaller diameter tail. The differences between both taxa are clearly shown, in the structure of the acrosome and nucleus. Therefore, according to our results and those reported in the literature, we indicate that Chilean native mussel sperm is different from other species of the Mytilus complex (M. trossulus, M. galloprovincialis and M. edulis). These differences in sperm ultrastructure found in M. chilensis, are another trait that can be used to validate the taxonomic status of the species. Differences in sperm morphology are related with reproductive isolation, and probably will be useful to understand future data on speciation. Finally, we discussed the finding that Mytilus galloprovincialis sperm from Chile have an acrosome notoriously smaller than those reported for specimens from Europe and Africa, though they have a great similarity with specimens from Japan, as reported in the literature.

Keywords: mussel, Mytilus chilensis, Mytilus galloprovincialis, sperm, ect-aquasperm, taxonomy.


RESUMEN. La ultraestructura de los espermatozoides se ha utilizado para resolver varios problemas sistemáticos y filogenéticos en invertebrados marinos. La ultraestructura de los espermatozoides del mejillón chileno Mytilus chilensis y Mytilus galloprovincialis corresponde al tipo acuesperma. La medida de los espermatozoides de ambos taxa fue de 55-60 μm entre la cabeza (acrosoma + núcleo), sector medio (solo 5 mitocondrias) y el flagelo, que en su pieza final tiene una cola de menor diámetro. La diferencia entre ambos taxa se mostró claramente en la estructura del acrosoma y el núcleo. Por lo tanto, de acuerdo a nuestros resultados y a los de la literatura, se indica que el mejillón chileno es diferente a los del complejo Mytilus (M. trossulus, M. galloprovincialis y M. edulis). Estas diferencias en la ultraestructura de los espermatozoides que se encontraron en M. chilensis, son otro rasgo que se puede utilizar para validar el estatus taxonómico de las especies. Las diferencias en la morfología de los espermatozoides están relacionadas con el aislamiento reproductivo y probablemente serán de ayuda para entender futuros datos de especiación. Finalmente, se discute que los espermatozoides de Mytilus galloprovincialis que se encuentran en Chile tienen un acrosoma notablemente menor a los reportados en especímenes de Europa y África, aunque tienen una gran similitud con los especímenes de Japón, como se reporta en la literatura.

Palabras clave: mejillones, Mytilus chilensis, Mytilus galloprovincialis, espermatozoides, acuesperma, taxonomía.


 

INTRODUCTION

Among the mollusks, bivalves of the Mytilidae family (Rafinesque, 1815) have external fertilization, therefore, the male gametes have a primitive morphological pattern type I or ect-aquasperm (Franzén, 1955; Jamieson & Rouse, 1989) with a conical head, underdeveloped midpiece and only one mitochondrial ring at the nuclear base, which has a single flagellum (Garrido & Gallardo, 1996; Kafanov & Drozdov, 1998).

Sperm morphology has been used to solve several systematic and phylogenetic problems in marine invertebrates (Franzén, 1955; Healy, 1988; Ferraguti & Gelder, 1991; Justine, 1991; Kafanov & Drozdov, 1998; Costa et al., 2004; Tyurin & Drozdov, 2005), moreover, the ultrastructure of sperm has been used with taxonomic purposes in several species (Popham, 1979; Franzén, 1983; Howard et al., 2009; Introíni et al., 2010). Initially it was used in mammals and then it was confirmed as valid for other animals including bivalve mollusks (Drozdov & Reunov, 1986). For example, it has been used to identify and classify species of the genus Mytella (Bivalvia) (Soot-Ryen, 1955), considering the presence of the axial rod (Introini et al., 2010), or to taxonomically differentiate species of the genus Bathypolipus (Cephalopoda) (Grimpe, 1921), using and comparing features of the acrosome (Roura et al., 2010). Also, Yurchenko (2012) indicated the species-specific differences in the sperm ultrastructure within the Ostreidae (Rafinesque, 1815), which could be identified both ultrastructurally and morphometrically. Therefore, this type of study is an accurate tool for the identification of marine species, especially if there are classification problems. Studies have demonstrated, particularly in mytilids, that the ultrastructure of the spermatozoa has a high taxonomic value, considering that the sperm characteristics of a species do not differ among different populations, because it may limit pre-zygotic reproductive isolation (Drozdov & Reunov, 1986; Hodgson & Bernard, 1986a, 1986b, Garrido & Gallardo, 1996; Kafanov & Drozdov, 1998; Introini et al., 2010).

Since 1976 there has been a significant increase in the taxonomic understanding of the Mytilus status, however, currently there is still confusion mainly regarding the species that live in the southern hemisphere of America. Mytilus chilensis (Hupé, 1854), a species endemic to Chile, found from Tirúa River (38°S) to Magellan Strait (53°S) (Hernández & González, 1976).

Although the taxonomic status of Mytilus chilensis has been recently reported (Ouagajjou et al., 2011) and genetic identification protocols for the Mytilus complex (M. edulis, M. trossulus, M. galloprovincialis and M. chilensis) have been published (Santaclara et al., 2006; Fernández-Tajes et al., 2011; Ouagajjou et al., 2011), there are still some studies indicating the opposite, classifying it as Mytilus edulis chilensis (McDonald et al., 1991; Toro, 1998b), Mytilus edulis platensis (Borsa et al., 2012) or Mytilus gallopro-vincialis chilensis (Cárcamo et al., 2005). Moreover, recent studies indicated that the Chilean mussel could correspond to a southern hemisphere lineage of M. galloprovincialis (Westfall & Gardner, 2010). This southern hemisphere lineage of the blue mussel Mytilus galloprovincialis has been diverging in allopatry from northern hemisphere conspecifics for about 0.84-1.2 million years (Westfall & Gardner, 2013). Notably, to date there is no consensus on the taxonomic validity of the Chilean mussels; however, studies of the spermatozoa ultrastructure in the Mytilus complex, including the Chilean mussel that can be useful as a taxonomic trait, have not been performed so far. On the other hand, the exotic species Mytilus galloprovincialis (Lamarck, 1819) which was originated in the Mediterranean Sea has recently been reported in Chile, Biobío Region (Daguin & Borsa, 2000; Toro et al., 2005) where it probably lives sympatrically with the Chilean native mussel. Moreover, it has been also recently demonstrated that laboratory crosses between these species generate viable hybrids, although larval survival differed with those from pure species (Toro et al., 2012).

The aim of this study was to compare the ultrastructure of the spermatozoa of the endemic Chilean mussel with the exotic Mytilus galloprovincialis from Caleta Tumbes, to provide information on both, its biology and the controversial understanding of the so called Mytilus complex.

MATERIALS AND METHODS

Mature males individuals of the Mytilus chilensis and Mytilus galloprovincialis were collected in Calbuco (41°49'S, 73°06'W) and Caleta Tumbes (36°43'S, 73°08'W), respectively, during spring time when there was a higher frequency of mature individuals (Oyarzún et al., 2011). Both mussel populations were subjected to genetic analysis to establish by molecular markers the differentiation of both taxa (Santaclara et al., 2006).

Samples for electron microscopy were obtained from freshly opened mussels. For transmission electron microscopy (TEM), small pieces of the testes were fixed in 2.5% glutaraldehyde in a 0.2-M phosphate buffer (pH 7.4) for 2 h at 4°C and post-fixed in 1% osmium tetroxide in a phosphate buffer for 2 h at 4°C. The pieces were then dehydrated in an ethanol series and embedded in araldite resin. Ultrathin sections were obtained on a Sorvall MT-1Ultra-microtome, stained with uranyl acetate and lead citrate (Glauert, 1965) and examined in an Hitachi H-700 transmission electron microscope. For the scanning electron microscope (SEM) analyses, 100 μm of sperm suspension from each of the five selected individuals were mixed (following Garrido & Gallardo, 1996), and a drop of sperm suspension was placed on a cover glass, prefixed with 2.5% glutaraldehyde in a 0.2-M phosphate buffer (pH 7.4) for 2 h at 4°C, and then post-fixed with 1% osmium tetroxide in a phosphate buffer for 2 h at 4°C. The material was dehydrated in a graded ethanol series, critical point dried, coated with gold, and then observed and recorded with a Leo-420 scanning electron microscope.

Size (Table 1) was measured in 50 spermatozoa obtained from a mixed sample of 12 individuals per species on the images obtained in the SEM, using the program © Image-Pro Plus v.6.0 for Windows (image analysis software).

Table 1. Acrosome length and nuclear diameter (μm) of the spermatozoon of Chilean mussels (Mytilus chilensis), M. edulis, M. galloprovincialis and M. trossulus ( ± DS) (n = 50).
 

For statistical differences between the two taxa in total length, the acrosome and nucleus diameter, was performed Student t-test for independent samples (Sokal & Rohlf, 1995). The presumptions of homoscedasticity and normality of variables and residuals of the model were tested by a Levene test and a Shapiro-Wilk test, respectively. The analyses were run using the statistical program SPSS v.15.0.1 (SPSS ibérica, IBM Company, Chicago, IL, USA).

RESULTS

The length of Chilean mussel's spermatozoa was 60.1 ± 0.07 μm and for Mytilus galloprovincialis it was 55.3 ± 0.08 μm, these measures comprised the three sperm regions: head, midpiece and the flagellum which in its end piece has a smaller diameter tail. In addition, there were no differences in structures between the two species (Fig. 2). Statistical analysis showed significant differences in the total length (t(98) = 335.69, P < 0.05), length of the acrosome (t(98) = -29.83, P < 0.05) and the nucleus diameter (t(98) = 9.68, P < 0.05), between the two taxa analyzed.

The male gamete showed, in both taxa, a conical head with an acrosome of elongated shape, with four areas of different density (Fig. 2). Chilean mussels presented a well developed acrosome (2.3 μm length) but it was smaller than the one of Mytilus galloprovincialis (3.1 μm). It is possible to observe in both mytilids the presence of an axial rod, which is a subacrosomal filament located in the endonuclear channel that extends from the subacrosomal material to the rear end of the midpiece and had 70 μm diameter (Fig. 2). Also, M. galloprovincialis shows a round nucleus of 1.70 μm diameter which is different to the oval nucleus of Chilean mussels of 1.88 μm diameter (Table 1, Fig. 2). Both species presented an underdeveloped midpiece with a single mitochondrial ring (both taxa 100% = 5 mitochondria) in the nuclear base. Inside are located a pair of centrioles, one proximal (the one nearer the nucleus) and one distal (Fig. 2). The presence of a single flagellum (9+2 microtubules, surrounded by a plasma membrane) stands out, which has a midpiece and an end piece or tail (Fig. 1).

 

 
Figure 1. Scanning electron micrographs of the mature sperm of a) Mytilus chilensis, and b) Mytilus galloprovincialis. a: acrosome, n: nucleus, f: flagellum, ep: end piece or tail.

 
Figure 2. Mature sperm TEM micrographs in longitudinal section of a) Mytilus galloprovincialis, and b) Mytilus chilensis. a: acrosome, ar: axial rod, av: acrosomal vesicle, br: basal ring, dc: distal centriole, m: mitochondria, n: nucleus, pc: proximal centriole, sm: subacrosomal material.

DISCUSSION

In this study, it was observed that the spermatozoa of both species had the same structures (Fig. 2) adjusting to the ect-aquasperm proposed by Jamieson & Rouse (1989). However, there were differences in the size and shape of some structures, mainly the acrosomal length and shape of the nucleus (Figs. 1, 2). These are considered species-specific taxonomic traits in the genus Mytilus (Popham, 1979; Crespo et al., 1990; Garrido & Gallardo, 1996; Kafanov & Drozdov, 1998). Therefore, Chilean mussels (M. chilensis) showed an acrosome of 2.3 μm length with a greater basal widening than M. galloprovincialis (Figs. 1, 2). It also showed a higher content of subacrosomal material and flattened basal rings that overlap (hood) the upper area of the nucleus (Table 1; Figs. 2, 3). Similar results were found for this species in the Corral Bay (Garrido & Gallardo, 1996). Therefore, according to our results and those reported in the literature, there is evidence that Chilean mussel sperm morphology is different from those species from the Mytilus complex (M. trossulus, M. galloprovincialis and M. edulis) (Table 1, Fig. 3). Hodgson & Bernard (1986a, 1986b) indicated that the use of the sperm ultrastructure of Mytilus edulis and M. galloprovincialis can be trusted for taxonomic purposes, because comparing gametes of these species with those from England, Japan (Niijima & Dan, 1965), North America (Longo & Dornfeld, 1967), South Africa and Spain (Crespo et al., 1990), they concluded that spermatozoa from mussels of all locations were species-specific (i.e., in structure and size), giving fidelity to the published scheme (Fig. 3). Later, Kafanov & Drozdov (1998) in a comprehensive review, used these evidence for a phylogenetic classification of the Mytiloida order. On the other hand, Westfall & Gardner (2010) reported genetic differences between species of Mytilus complex and Chilean mussel with 16S rRNA RFLP assay; similarly Ouagajjou et al. (2011) found similar results by using nine microsatellite. Thus, although between these four species, differences at the spermatic and genetic level exist, it is still necessary to understand the evolutionary history of Chilean mussel. It would be interesting to compare the morphology of spermatozoa with phylogenetic data to observe species-specific changes in the germ cells.

 
Figure 3. Structural pattern of spermatozoa of the a) Chilean mussels (present study), b) M. edulis (extracted image Hodgson & Bernard, 1986a, 1986b), c) Mytilus galloprovincialis (present study), and d) Mytilus galloprovincialis (extracted image Hodgson & Bernard 1986a, 1986b). We use the same format of the original schematics for better comparison.

It is very important to elucidate the taxonomic status of the Chilean mussel, mainly because of its high importance as economic resource in the south of Chile (Oyarzún et al., 2011; SERNAPESCA, 2012). In addition, the designation of origin, that is a geographical indication applied to food products (e.g., mussels), have as starting point, the taxonomic status, an issue that Chile is approaching in some endemic species that will strengthening the product, considering in special the Chilean mussel which is actually in the third place among global mussel production (Freire et al., 2007; FAO, 2012).

Previous studies have determined that spermatozoa of Mytilus galloprovincialis in populations of England, Spain and South Africa (Hodgson & Bernard, 1986a,1986b; Crespo et al., 1990) are similar in structure, form and size regarding both the total and acrosomal lenght (50 μm and 5.1 μm, respectively) and the number of mitochondria (between 5 and 6). However, our results showed a significant difference in the acrosome length (3.1 μm) and in the number of mitochondria (100% = five) (Table 1). Komura et al. (1995) reported in Japan, features and size in spermatozoa of this species, equal to those found in the present study (Table 1, Fig. 3). Accordingly, it is possible to infer that both populations (Japan and Chile) share sperm features that are different from those reported in Europe and South Africa. Unfortunately, the studies conducted in Europe and South Africa did not used molecular markers to identify taxa, and probably some mussels analyzed were hybrids; especially taking into account the existence of the hybrid zone in England (Wilhelm & Hilbish, 1998). Moreover, Briones et al. (2012) found differences in the acrosome length (between 1.01 μm and 2.52 μm) among populations of the intertidal mussel Perumytilus purpuratus, along a latitudinal gradient of ~2200 km, and indicated that is probably due to a speciation process. This process occurs due to differences in the acrosome and/or flagellum which produce prezygotic incompatibility (between sperms and eggs), because there are receptors located in these structures that are used for fertilization in some marine invertebrates, especially those with external fertilization (Pitnick et al., 2009). In this context, although there have been published studies indicating gamete incompatibilities among species of the Mytilus complex (Bierne et al., 2002; Rawson et al., 2003), this is the first study that finds evidence which indicates quantitative differences between Chilean mussel sperm with other species of the Mytilus complex. However, the role that sperm morphology plays in fertilization is surprisingly poorly known (Howard et al., 2009). While the role of the acrosomal protein M7 lysin on reproductive isolation in Mytilus is well understood, the mechanisms driving the evolution of this protein are not jet fully elucidated (Springer & Crespi, 2007; Hess et al., 2012).

The difference in sperm ultrastructure found in M. chilensis with regard to M. edulis and M. galloprovincialis is another trait, which can be used to validate the taxonomic status of the former species. Clearly our results provide an interesting line of research to understand speciation and reproductive isolation, as a fundamental process in evolution (Mayr, 1969; Wiley, 1977). Considering the recent divergence among Mytilus species complex (in the Pleistocene between 0.84 to 1.2 mya (Gérard et al., 2008)), this evidence could of help to understand the early morphological changes in the germ cells and could answer: (1) What are the morphological changes in sperm that produce a reproductive barrier? or, (2) What are the genetic and molecular underpinnings of prezygotic reproductive isolation?

ACKNOWLEDGEMENTS

The authors would like to especially thank Marcela Riveros for the drawing of schemes for this study. This study was financed by the project FONDECYT 1120419 of JT and project CODEI-31-2010 of RG.

 

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Received: 7 June 2013; Accepted: 30 December 2013

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