CYTOGENETIC STUDY OF Diplectrum eumelum Rosenblatt and Johnson, 1974 (PISCES: SERRANIDAE)

Trabajo recibido el 11 de enero de 1990 y aceptado Para su publicación el 6 de marzo de 1990.

RESUMEN

INTRODUCCIÓN

The classification into the Serranidae Family is clearly insatisfactory, owing this to the similar characteristics presented by the organisms which are considered properly Serranidae and their close relatives, in such a manner that the location in the different genera is based more frequently in the absence of common specific characteristics than to the presence of basic similarities (Smith, 1965).

The Serranidae is one of the bigger and diversified families of the teleost fishes. lt is located among other 64 families in the Percoid Superfamily (Nelson, 1984); nevertheless, these families show general characteristics that make difficult lo stablish the limits among them. In general it is located in the base of the evolution of Percoidei Suborder and it has even been used as a "lay aside" taxonomical entity which includes near 35 genera and about 370 species: all the lower percoidei poorly envolved fishes (Gosline, 1966 and Nelson, 1984). For this reason the organisms which integrate the Percoidei Suborder have been subjet of various systematic reviewings in order to get a solution to this complex problem (Gosline, 1966, Smith, 1971, Kendall, 1979, Randall 1980).

The Diplectrum genus (Holbrook: 1855 - 32) which has been very well studied, reviewed and characterized by Bortone (1977) and previously by Rosenblatt and Johnson (1974) is located in the Serranidae Family. There has been a great confusion in the literature in order to located the organisms and the different species, taking in to account the evident morphoanatomic and meristic homogenity, and the consequent need to use characters submited to wide variation as coloring (of fin nose, operculum, and caudal pedunculus), number of scales (on cheeks and lateral lines), gillspines and pseudogill filaments; a mistake in the "weight" given to this characteristic may result, in consequence, in an erroneous determination of the species.

However, it is considered that the Diplectrum genus is formed by nine endemic species in the Pacific Ocean East Coast and three in the Atlantic Ocean West Coast, (tropical or subtropical). Its species are hermaphrodites and in some cases the authors propose the recognizing of subspecies as in the case of D. formosum(Bortone, 1977 and Darcy, 1985).

Due to the differences between the species are very subtle, and in some cases the distinctive characters are overlapping, the objective of the present study it is to stablish the karyotype ofDiplectrum eumelum contributing to its characterization, in order to help to define the systematic location of the group and the Serranidae Family into the evolutionary context.

The studied organisms were collected and processed on board of the B/O "El Puma" vessel during the oceanographic campaigns CAPECAL II, III and IV, which took place in september - october of 1984, 85 and 86, respectively. The specimens were caught in the Pacific Ocean area located between the 28°19.7'-21°18.4' North Latitude and the 105°32.5'-111°34.8' West Longitude (Fig. 1).

Fifteen hermaphrodites organisms were processed by the cytogenetical techniques for gill epithelium which were established by McPhail and Jones (1966), Denton (1973) and Kligerman and Bloom (1977).

The metaphases which showed a complete chromosomic complement, high definition of chromosomes and adequate concentration of chromatine (not very high) were chosen in order to stablish the karyotype. Those fields with overlapping chromosomes were omited.

The chromosomes were classificated by the methods proposed by Levan et al. (1964) and Al -Aish (1969).

A number of 98 metaphases with different levels of chromosome spiralization were obtained. In all cases they showed a diploid number (2n) of 48 chromosomes. Chromosomic heteromorphism or negative heteropygnosis which makes evident the presence of sexual chromosomes, was not found (Fig. 2).

Likewise, 18 mitotic fields obtained from organisms presented the desired characteristics for their analysis, showed two pairs of submetacentric, one pair of metacentric and 21 pairs of telocentric (acrocentric) chromosomes (Tabla 1).

The karyotype and Idiogram of Diplectrum eumelum are characterized by: a diploid number (2n) of 48 chromosomes, a haploid number (n) of 24, a Karyotype formula of 2sm + 1 m + 21a and major chromosome arm number of 54 because three of its elements are biarmed an 21 monoarmed (Figs. 3 and 4).

Figure 1. Collecting sites of Diplectrum eumelum corresponding to Oceanographic campains CAPECAL II, III and IV.

Figure 2. Mitotic Fields of Diplectrum eumelum (2n=48).

TABLE 1 RESULTS OF THE STATISTICAL ANALYSIS OF THE KARYOTYPES CHROMOSOMES OF Diplectrum eumelum

Figure 3. Karyotype of Diplectrum eumelum Rosenblatt and Johnson, 1974.

Figure 4. Idiogram of the Karyotype of Diplectrum eumelum.

Taking in to account that the biarmed elements (metacentric and submetacentric) come from the monoarmed chromosomes (acrocentric: telocentric), several authors (Ohno et al., 1968, Denton, 1973, Thompson, 1979 and Uyeno et al., 1983) consider that the karyotype of the ancestral teleost fishes was of 48 achrocentric chromosomes. The antecedents about cytogenetic: works of the perciform fishes are scarce.

The most studied families from this point of view are the Centrarchidae (23 species) and Cichlidae (41 species) but only one species has been studied from the Serranidae Family (Denton, 1973, Thompson 1979).

The diversity of the Serranidae Family has made difficult its study, nevertheless it is very interesting from the taxonomic and evolutionary points of view. As an example, the genusHypoplectrus forms a group of 12 hermaphroditic: morphospecies which are sinchronic in the new world and differ among them in the patterrn of coloring but a little in morphometry, meristic, ecology or reproductive behavior. For this reason Graves and Rosenblatt (1980) made the electrophoretic analysis of seven different kinds of colored fishes of this genus located in the same reef: the conclusion was that there are not enough arguments in order to recognize more than one species, because it is a clear example of a polimorphic population; notwithstanding that at the same time Fischer (1980), by means of this "SCUBA" observations, conclude that it was a case of an multispecific complex (Nelson, 1984).

From this perspective, the morphoanatomic homogenity of the genusDiplectrum is not a "hig weight" to locate the organisms in different species. For this reason more analysis is needed, taking to account that there are few precise characters in order to achieve a correct taxonomic determination, besides the interesting reviews of Bortone (1977) and Rosenblatt and Johnson (1974) related to Diplectrum eumelum as a recent taxonomic species. Some research in the Gulf of California and the Pacific Ocean done by Castro - Aguirre et al. (1970), Heiden (1985) and Pérez -Mellado and Findley (1985), show the variations which can be present in the species composition of Diplectrum genus, and thus the evident need for defining these taxonomical entities.

According to the classification of the Serranidae family and its sexual patterns, which were analized by Smith (1965) and Smith and Young (1966), the Serranus genus is considered one of the most primitive of the sinchronic hermaphroditic Serranidae as well as theHypoplectrus and Diplectrumgenera.

There are only two cytogenetic antecedents of the Serranidae Family which are the Centropristes ocyurus study with a diploid number of 48 chromosomes: 28 metacentric and 20 submetacentric (Durán-González and Laguarda-Figueras, 1987) and theCoreoperca kawamebari studys with 48 acrocentric chromosomes (Nogusa, 1960 and Denton, 1973). In the present time some genera of this Family are in an uncertain taxonomical location. The Coreopercagenus, was located in the Perichthydae Family by Gosline (1966), which is still subjects to change in its composition (Nelson, 1984).

Apart from the results obtained in the Diplectrum eumelum study which is characterized by a diploid number of 48 chromosomes, 4 submetacentric, 2 metacentric and 42 acrocentric, there is information of the karyotype ofSerranus phoebe that shows 48 acrocentric chromosomes (study in progress). The results are similar to those obtained by Smith and Young (1966) through their analysis of the Serranidae patterns of sexuality.

The Centropristes genus has its own morphologic characteristics, such as the shape of the caudal fin, the pelvic fins and an accentuated sexual dimorphism. Notwithstanding, it is a protogenic hermaphrodite whose skull morphology presents certain similarities that suggest affinity with Paralabrax, a secondary gonocoric. According to this, it is considered that both come from a common ancestral and that difference between their sexual mechanisms makes evident that they must be recognized as separated genera (Smith and Young, 1966). Also, the cytogenetic antecedents of Centropristes ocyurus show a karyotypic evolved genus by the presence of its biarmed chromosomes.

According to their cytogenetic and karyotypic antecedents, these genera can be located from the most to the less evolved, being ordered as follows: Serranus, Diplectrum andCentropristes.

The karyotype of Diplectrum eumelum is characterized by a diploid number of 2n = 48 (n = 24). The analysis of the complement shows the existence of one metacentric, 2 submetacentric and 21 acrocentric pairs, with a mayor chromosome arm number of 54 (n. f. = 54).

In accordance with the cytogenetic studies, it is suggested that in the Serranidae family the Diplectrum genus is less evolved than Centropristes and more evolved thanSerranus.

Taking in to account the significant specialization of the reproductive systems presented by the Serranidae family, the cytogenetic and electrophoretic studies are very important to know which differences are significant in order to get a better taxonomic ordaining.

The authors are grateful to Dr. Antonio García-Cubas and to M. in C. Felipe Flores, from the "Instituto de Ciencias del Mar y Limnología (ICMyL)" of the "Universidad Nacional Autónoma de México (UNAM)" for the help they gave us in order to participate in the oceanographic campaigns CAPECAL.

In like manner to the M. in C. Margarito Alvarez, from the ICMyL, and to the M. in C. Patricia Fuentes and the Biól. Héctor Espinosa from the "Instituto de Biología" of UNAM for the valuable help in the classification of the biologic material.

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