Retina‐specific LDH isozyme in blueback herring, Alosa aestivalis (Mitchell), and alewife, Aiosa pseudoharengus (Wilson)
Authors: McKenzie Joseph A. 
Published: August 2010

The retina of 390 Alosa aestivalis and 410 Alosa pseudoharengus have been examined by means of starch‐gel electrophoresis. The retina‐specific E4 isozyme has been found to occur in all the fish examined. This study demonstrates for the first time that the E4 isozyme occurs in A. aestivalis. Because the E4 isozyme is not polymorphic and has an identical mobility in A. pseudoharengus and in A. aestivalis it is neither suitable for use as a species identification characteristic nor a population marker.

Alosa aestivalis and Alosa pseudoharengus are two commercially important ana‐dromous species of fish in New Brunswick, Canada. These two species may occur together in the same spawning runs but wrhile A. pseudoharengus has a wide distribution along the East coast of North America (Leim & Scott, 1966) A. aestivalis occurs in a very limited area in Canada where it is at the northern limit of its range and because of increasing threat of pollution has been listed by McAllister (1970) as one of 17 endangered species. These two species of fish are morphologically very similar and can only be separated by the colour of the abdominal peritoneum (Leim & Scott, 1966). McKenzie (1973) has compared these two species by means of protein electrophoresis and found that while the muscle myogen patterns were species specific, the LDH patterns were the same in both species. He described these two species as five isozyme fish showing the LDH isozymes A4, A3B, A2B2, AB3 and B4. Since Horowitz & Whitt (1972) have reported the presence of the E4 isozyme in the retina of some teleosts including. A. pseudoharengus but not including A. aestivalis, I considered it worth‐while to re‐examine A. aestivalis and A. pseudoharengus to find out whether A. aestivalis possessed this isozyme and, if so, whether the mobility of the isozyme could be used as a species identification characteristic and as a population marker.

The fish used in this study were collected from five different locations during the 1971 spring migration period and held deep frozen for eight months before they were examined. They were identical to the specimens used for the study reported by McKenzie (1973) where the collection dates, numbers of fish and geographic locations are given.

One eyeball from each of 390 A. aestivalis and 410 A. pseudoharengus was removed. Each eyeball was homogenized in 10 drops of distilled water and allowed to stand for one hour at 4oC. The samples were then centrifuged for 10 min at 12 000 g. The supernatants were used immediately for vertical starch‐gel electrophoresis. The apparatus used was that described by Boyer & Hiner (1963). The conditions of electrophoresis were the same as used by Saunders & McKenzie (1971). The LDH bands were stained by the deposition of blue formazan dyes in the regions of LDH activity. The stain formula and details of methods are given in Whitt (1970).

The LDH isozyme patterns of all the fish examined were identical. The location of the isozymes A4, A3B, A2B2, AB3 and B4 are indicated in Fig. 1. A4 is abundant in muscle while B4 is abundant in heart. Because the LDH subunits assemble preferentially into homodimeric pairs before forming tetramers (Markert & Ursprung, 1971). A4. A2B2 and B4 show up in electropherograms as strong bands while A3B and AB3 show up as weak bands. The retina‐specific isozyme E4 is shown between B4 and AB3. Whitt (1970) has already demonstrated that A. pseudoharengus is a five isozyme fish. This has been confirmed by McKenzie (1973) who also compared both A. pseudoharengus and A. aestivalis and found these five isozymes had identical mobilities in both species of fish. The present study demonstrates for the first time that the retina‐specific E4 occurs in A. aestivalis where it has the same electro‐phoretic mobility as that of A. pseudoharengus. The patterns are the same and do not appear to vary with geographic origin of the fish.

The reason why the presence of the E4 isozyme was demonstrated in the present study and not in the previous one (McKenzie, 1973) is because of the method used. In that study the migration length was not long enough to sufficiently separate the enzymes. In the present study vertical starch‐gel electrophoresis which allows for long migration distances was used. As has already been shown for the A‐B isozymes (McKenzie, 1973), the E4 isozyme is not polymorphic in these two species of fish. It therefore has no use as apopulation marker. Because the E4 isozyme has an identical mobility in both species of fish, it cannot be used as a species identification characteristic.