J. Fish Biol. (1978) 12, 173-179

Karyotypic analysis of the gobiid fish genus Quietula Jordan and Evermann

PETER C . COOK* Department of Ecology and Evolutionary Biology, The University of Arizona,

Tucson, Arizona 85721, U.S.A.

(Accepted 10 February 1977)

The karyotypes of the two species of the gobiid fish genus Quietulu, Q. y-cuudu (Jenkins & Evermann) and Q. guuymusiue (Jenkins & Evermann), are reported for the f ist time. The karyotypes contained equal numbers of chromosomes (2n=42) but differed in chromo- some morphology. Quietulu y-cuuda has 42 acrocentric chromosomes. Quietulu guuy- masiue has six metacentric, four submetacentric, and 32 acrocentric chromosomes. It is suggested the karyotype of Q. guuymasiue might have been derived from that of Q. y-cuudu. In addition, from comparison of the species’ karyotypes of the genera Quietulu and Gillichthys, it is possible the karyotype of Q. y-cuudu might have been derived from that of Gillichthys mirubilis.

I. INTRODUCTION

The karyotypes of the two species of gobiid fish Quietula y-cauda (Jenkins and Evermann, 1888) and Q . guaymasiae (Jenkins and Evermann, 1888), are reported here for the first time. The data are used in discussing the status at species level, the evolutionary relationship of the two taxa, and the evolutionary relationship of the genera, Quietula and Gillichthys.

The karyotypes of Gillichthys mirabilis Cooper 1864, and G. seta (Ginsburg, 1938), the only species in that genus, were previously reported by Chen & Ebeling (1971). They were able to hypothesize from analysis of the evolution of the karyotypes the derivation of G. seta from G. mirabilis. Earlier, Barlow (1961) had proposed this derivation from analysis of the morphology and ecology of the two species. The possibility that the karyotypes and the mode of evolution of the karyotypes of the Quietula species may be similar to those of the Gillichthys species was presumed because of the close similarity between these genera.

Similarities exist between the genera in number of species, distribution, ecology, and morphology. Each genus has two species, one of which is endemic to the upper Gulf of California, the other occurring on the Pacific coast of California and Baja California as well as in the upper Gulf. Three of the species live in estuaries but G. seta inhabits rocky inter-tidal pools located near the mouths of estuaries (Barlow, 1961). Morphological similarities have been recognized and reported by several workers, including Jenkins & Evermann (1888), Gilbert (1889), Jordan & Evermann (1896), and Ginsburg (1945).

Gillichthys mirabilis and Quietula y-cauda occur sympatrically in the estuaries along the Pacific coast of California and Baja California, and in the upper Gulf. Quietula guaymasiae and Gillichthys seta are endemic to the upper Gulf and may be found sympatrically with the other species.

* Present address: Aqua Finca de Camarones Aquaculture Farms Ltd., San Pedro Sula, Honduras. 173

174 P. C. COOK

II. MATERIALS AND METHODS Collection data for the fishes used in this study are given in Table I. The collections were

made with a beach seine in three estuarine habitats along the Sonoran coast of the Gulf of California. The fishes were transported to Tucson, Arizona, in plastic bags containing a small amount of sea water saturated with oxygen. Despite frequent collecting efforts in suitable habitats only one sample of Quietula y-cauda was available for study.

TABLE I. Collection data for specimens of Quietula guaymasiae and Quietula y-cauda used in the preparation of karyotypes. All collections are from the Sonoran coast of the Gulf of

California

COIL date Species Locat ion Size range Sex ratio

(sL., mm) (M : F) n

Quietula guaymasiae June 1973

Summ. 1973

Sept. 1973

Feb. 1976

Quietula y-cauda Feb. 1976

Bahia Kino; c. 1 mile N. 3 56.0-65.0 3 : 0 of Pta. Ignacio (28" 53' N; 112" 2' W) Pto. Peiiasco; Bahia 2 42.0-46.0 0 : 2 Cholla, tide channels (31" 22"; 113" 40'W)

N. of Bahia Cholla (31'22"; 113'40'W) Guaymas; Estero Soldado 2 34.0-35-5 2 : 0 (27'57"; 1ll"W)

(27'57"; 111" W)

Pto. Peiiasco; c. 1 mile 4 46'0-59.0 2 : 2

Guaymas; Estero Soldado 8 30.0-39.0 0 : 8

The karyotyping procedure used is basically that of Fisher & Rachlin (1972). The source tissue used for the karyotypes was gill epithelium. The procedure involves the following steps: pretreatment with colchicine, hypotonic treatment, tissue fixation, staining and mounting, and microinspection.

Colchicine is a mitotic inhibitor which arrests cell division at the metaphase stage. Two methods of administration were used: intramuscular injection and total immersion bath. The bath was used according to the method of Chen (1967), for individuals too small to be injected. The dosage and concentration of the colchicine dissolved in distilled water varied (Table 11) within the range of amounts reported as successful by Denton (1973). The length of the pretreatment period was varied (Table 11) to test the relative effects of the periods of exposure to colchicine on the quality and quantity of chromosome spreads produced. The results of this test are indicated as positive or negative (unsatisfactory products) in Table 11. The negative results may be partially due to inadequate staining with Giemsa stain which was not maintained at the proper pH 6.4 (Denton, 1973).

The fixative solution differed from that of Fisher & Rachlin (1972) by the use of methanol instead of ethanol. Denton (1973) states that methanol is preferred by most workers.

Chromosome preparations mounted on microscope slides were observed with a light microscope with a green filter at oil immersion magnification ( x 950). Photographs were made with a 4 x 5 inch camera using Kodalith Ortho Type I11 film and Kodabromide F-5 paper (Eastman Kodak Co.).

Chromosome counts were made with a microscope. In cases in which the chromosomes were crowded or overlapped, photographs were helpful in identifying individual chromo- somes.

The chromosomes were classified according to Chen (1967). This classification recognizes three morphotypes based on the ratio of the short arm of the chromosome to the long arm. The three morphotypes are: metacentric (ratio >0.9), submetacentric (ratio >0.3, but

KARYOTYPIC ANALYSIS OF THE GOBIID FISH GENUS 175

TABLE 11. Procedural variations in colchicine pretreatment per individual fish (listed in chronological order of usage). Preparations producing satisfactory results are indicated by

' + ' under ' Usable Material ' -

Dosage Concentration Pretreatment Usable (ml) colchicine sol. length (h) material Species Method

Quietula guaymasiae Injection Injection Injection Injection Injection Injection Injection Injection Injection

Quietula y-cauda Bath Bath Bath Bath Bath Bath Bath Bath

0.10 0.10 0.10 0.10 0.10 0.02 0.02 0.01 0.02

250.00 250.00 250.00 250.00 250.00 250.00 250.00 250.00

0.05 % 0.05 % 0.05 % 0.05 % 0.05 % 0.50 % 0.50 % 0.50 % 0.50 %

0.02 % 0.02 % 0.02 % 0.02 % 0.02 % 0.02 % 0.10 % 0.10%

24 6

24 2 4 3

15 24 24

+ -

+ + + -

4 5 + 4.5 +

15 + 15 + 11 + 11 + 5 + 5 +

Quietula guaymasiae Bath 250,oo 0.10% 5 + Bath 250.00 0.10% 5 +

<0.9), acrocentric (ratio < 0 3 ) . The ratios were determined from measurements made with dial calipers on enlarged photographs. Metacentric and submetacentric chromosomes are considered biarmed chromosomes. Acrocentric chromosomes are considered uni-armed. Karyograms were made from the best photographs.

III. RESULTS

Of the 19 individuals 14 provided satisfactory material. Lack of usable material from the other five might be explained by the variations employed in the colchicine pretreatment period or poor staining. The 14 individuals providing the karyotypes were eight females of Quietula y-cauda and two females and four males of Quietula guaymasiae. For Quietula y-cauda, 109 chromosome spreads were analysed, and for Q. guaymasiae, 188 chromosome spreads were used.

The diploid number of both species of Quietula is 42. The karyotypes are dis- tinguishable on the basis of chromosome morphology. The karyotype of Quietula y-cauda has 42 acrocentric chromosomes (Fig. 1). The karyotype of Quietula guaymasiae has 32 acrocentric, four submetacentric, and six metacentric chromo- somes (Fig. 2). The chromosome arm numbers (n .5 ) are 42 and 52, respectively. There are no obvious sex chromosomes in Q. guaymasiae, and I assume that there are none in Q. y-cauda (no male Q. y-cauda were caught).

IV. DISCUSSION

With few exceptions it is generally recognized that every species has a unique

176 P. C. COOK

karyotype (White, 1973). The karyotypes of Quietula y-cauda and Q. guaymasiae are distinctly different. Therefore, even though Gilbert (1889) and Jordan & Evermann (1896) were doubtful of the status of the two species on the basis of external morph- ology, the karyotype data support their recognition as separate species.

The evolutionary relationship of the two species may be analysed from the karyo-

c %

o o L n * . a f i I ) - FIQ. 1. Karyotype and karyogram of Quietula y-cauh (2% aceto-orcein). Acro=acrocentric.

types. The evolution of fish karyotypes, and of gobiid fishes in particular, seems to follow a trend toward a smaller complement with a greater proportion of biarmed chromosomes (Chen, 1967; Chen & Ebeling, 1971). Both species have 42 chromo- somes but Q. guaymasiae has a greater proportion of biarmed chromosomes. This suggests the karyotype of Q. guaymasiae is more recently derived than that of Q. y-cauda. I conclude, therefore, that Q. guaymasiae might have been derived from Q. y-cauda or a y-cauda-like ancestor. The derivation of the karyotype of Q . guay- masiae from the karyotype of Q. y-cauda must have involved, at least, ten pericentric inversions.

KARYOTYPIC ANALYSIS OF THE GOBIID FISH G E N U S 177

Comparison of the karyotypes of Quietula and Gillichthys (Table 110, using the same criteria, suggests that, since they are closely related genera, the karyotype of Quietula y-cauda might have been derived from the karyotype of Gillichthys mirabilis or a mirabilis-like ancestor. This derivation of karyotype must have involved a reduction in chromosome number from 44 to 42. This might have occurred by centric fusions in, at least, four acrocentric chromosomes forming two biarmed chromosomes.

G Ir

cl

O a a a FIG. 2. Karyotype and karyogram of Quietula guaymsiae. (Giemsa) Meta.=metacentnc, Smet.=

submetacentric, Acro. = acrocentric.

Since the karyotype of Q. y-cauda does not contain biarmed chromosomes, those formed as a result of centric fusions, or any that might have been present in the ancestral karyotype, must have been transformed subsequently into uni-armed chromosomes by pericentric inversions (at least 14). The reduction in chromosome number might also have occurred by tandem fusions of two pairs of acrocentric chromosomes, thereby not forming biarmed chromosomes, but requiring, at least, 12 pericentric inversions to convert the existing biarmed chromosomes of the ancestral karyotype to acrocentrics. The karyotype of Quietula y-cauda could have been derived by either of these mechanisms from the karyotype of Gillichthys mirabilis.

178 P. C. COOK

TABLE 111. Chromosome rnorphotype classification of the karyotypes of Quietula y-cauda, Q. guaymasiae, Gillichthys mirabilis, and G. seta

Species 2n Meta. Smet. Acro. n.f.

Q. y-cauda 42 0 0 42 42 Q. guaymasiae 42 6 4 32 52 G. mirabilis* 44 0 12 32 56 G. seta* 44 6 14 24 64

*Chen & Ebeling, 1971.

On the basis of the karyotype information presented here, and the speciation model described for Gillichthys seta by Barlow (1961), I suggest that Quietula y-cauda was derived from Gillichthys mirabilis or mirabilis-like ancestor in the estuaries on the Pacific coast of California and Baja California. These species entered the Gulf of California together at a time when their ranges were displaced southward by lowering temperatures (Walker, 1960). When the waters warmed again their ranges were displaced northward, separating the Gulf populations from the outer coast popula- tions, until they reached their present locations in the upper Gulf. It is presumed that Quietula guaymasiae and Gillichthys seta speciated there.

Similar processes of evolution may be found again in the genus Zlypnus. One species, Zlypnus gilberti occurs sympatrically with Quietula y-cauda and Gillichthys mirabilis throughout their ranges. Two other species, Zlypnus luculentus (Ginsburg, 1938) and an undescribed species, are endemic to the Gulf of California. Analysis of these species’ karyotypes may show a direct relationship of Ilypnus to Quietula and Gillichthys.

This paper is a modification of my MSc. thesis submitted to The University of Arizona. My Guidance Committee included Drs Donald A. Thomson, Charles H. Lowe, and Oscar G. Ward. They helped in the design of the study and provided space and materials with which to do the work. Thanks are due40 P. S. Miles, C. E. Lehner, and L. T. Findley for helping to collect specimens. This is a contribution of the Marine Biology Program of The University of Arizona.

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K A R Y O T Y P I C ANALYSIS OF THE GOBIID FISH G E N U S 179

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