Den sity-de pend ent reg u la tory mech a nisms of the gen er a tive po tency in the males of a bank vole Myodes glareolus

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1 Acta Theriologica 53 (0): , PL ISSN Den sity-de pend ent reg u la tory mech a nisms of the gen er a tive po tency in the males of a bank vole Myodes glareolus Vera MAMINA and Oleg ZHIGALSKY Mamina V. and Zhigalsky O Den sity-de pend ent reg u la tory mech a nisms of the gen er a tive po tency in the males of a bank vole Myodes glareolus. Acta Theriologica 53: Mor pho log i cal anal y sis re vealed de struc tive changes in the testicular tis sue of bank voles Myodes glareolus Schreber, 1780 dur ing dif fer ent phases of a pop u la tion fluc tu a tion. The most pro nounced changes were ob served at the peak phase, when karyolysis of the Leydig cells and de gen er a tion of spermatogenous cells were re corded in 90% of males. Dur ing the in crease phase, de pres sion of androgenous testicular func tion was ob served in only 50% of males and at ro phy of seminiferous tu bules in 30% of males. Dur ing the low phase, the pro por tion of males with de struc tive changes in gen er a tive and en do crine por tions of the tes ti cles did not ex ceed 30%. Morphometrical anal y sis of sper ma to zoa dem on strated that the size of the head and nu cleus were re lated to the phase of the pop u la tion fluc tu a tion. Lengths of the mid dle and main parts of the sper ma to zoon tail, as well as the size of the acrosome, were not re lated to phase of the pop u la tion fluc tu a tion. In sti tute of Plant and An i mal Ecol ogy, Ural Di vi sion of Rus sian Acad emy of Sci ences, 8 Marta 202, , Ekaterinburg, Rus sia, mamina@ipae.uran.ru Key words: den sity, de struc tive changes, morphometrical anal y sis, phase of pop u la tion num bers, population cycles Introduction Many spe cies of arvicoline (microtine) ro - dents un dergo high-am pli tude fluc tu a tions in abun dance. Some pop u la tion fluc tu a tions are short-term, com plet ing a fluc tu a tion within a few months (Krebs and Myer 1974, Taitt and Krebs 1985), whereas oth ers may take 2 3 years to run their course (Oksanen and Henttonen 1996). The in ter val be tween pop u la tion fluc tu a - tions, whether short-term or en com pass ing 2 or more years, may be an nual or er ratic; fluc tu a - tions that oc cur at 2 5 year in ter vals may be de - scribed as pop u la tion cy cles (Krebs and Myers 1974, Taitt and Krebs 1985, Krebs 1996, Bj rnstad et al. 1998). Pop u la tion den sity is de ter mined by birth rate, im mi gra tion, mor tal ity, and em i gra tion (Zhigalsky 1992, Klemola et al. 2002, Norrdahl and Korpimäki 2002, Huitu et al. 2004, Cerqueira and De Sousa 2006, Getz et al. 2006, 2007). Hy poth e ses pro pos ing mech a nisms in - volved in these pro cesses can be di vided into two groups: (1) ex trin sic fac tors such as me te o ro log i - cal events, food sup ply, and pred a tor abun dance [1]

2 2 V. Mamina and O. Zhigalsky that reg u late birth rate, im mi gra tion, mor tal ity, and em i gra tion; (2) in trin sic fac tors, in clud ing so - cial be hav iour, such as territoriality, and other so cial in ter ac tions re sult ing in vari a tion in the qual ity of an i mals that af fect re pro duc tion and mor tal ity, as de ter mined by pop u la tion den sity. Al though pop u la tion dy nam ics and the mech a - nisms driv ing cy clic fluc tu a tions have been ex - ten sively stud ied, the causes and mech a nisms driv ing this phe nom e non re main un known. Long-term cy clic fluc tu a tions in pop u la tion den - sity and the mech a nisms in volved are among the ma jor enig mas of pop u la tion ecol ogy. Data on bank vole Myodes glareolus Schreber, 1780 pop u la tion dy nam ics were used to de ter mine whether the ob served fluc tu a tions were sto chas tic or un der lain by bi o log i cal mech - a nisms. We con ducted a long term study of dy - nam ics of a bank vole pop u la tion in the subtaiga zone in the Mid dle Urals. This pop u la tion underwest pe ri odic fluc tu a tions, with in ter vals of one and three years. The one-year fluc tu a - tions were re sponses to sea sonal changes in en - vi ron men tal fac tors. Fluc tu a tions with a three-year in ter val were de ter mined by in trin sic den sity-de pend ent mech a nisms (Bernshtein et al. 1989, Zhigalsky and Kshnyasev 1999, Zhugalsky and Kshnyasev 2000). In ex cep tion ally dense pop u la tions of small ro dents, es pe cially in en clo sures and cages, ag - gres sive in ter ac tion be tween an i mals re sult in in creased adrenocortical ac tiv ity (Chris tian 1971, 1974). The in creased cir cu lat ing corticosteroids in hibit the re pro duc tion pro cess and re duce sur vival. Sim i lar phe nom ena have been ob served in free-liv ing pop u la tions of Microtinea (Lee and Mc Don ald 1985, Aguilera and Rabadan-Diehl 2000). Sea sonal stress has an in hib i tory ef fect on male re pro duc tive func - tions, which are me di ated in part by di rect ac - tions of glucocorticoids on Leydig cells in the tes tes (Hardy et al. 2005). Leydig cells are the pri mary source of tes tos ter one in males. Lev els of cir cu lat ing tes tos ter one are de ter mined by the steroidogenic ca pac i ties of in di vid ual Leydig cells and the to tal num ber of Leydig cells in the tes tes. Stress-in duced in creases in se rum glucocorticoid con cen tra tions in hibit tes tos ter - one-biosynthetic en zyme ac tiv ity, lead ing to de - creased rates of tes tos ter one se cre tion. High lev - els of intratesticular tes tos ter one se creted by the Leydig cells are nec es sary for spermatogenesis. In suf fi cient go nad o tro pic hor - mones pro voke a de lay or dis tur bance in the spermatogenetic pro cess that is ac com pa nied by de struc tive changes, af fect ing both gen er a tive and en do crine as pects of the tes tes (De Kretser 1984, Mamina and Zhigalsky 2004). Sea sonal reg u la tion of testicular func tion in mam mals is a well-known phenomenon (Blottner et al. 1999, Blottner et al. 2000, Gockel and Ruf 2001, Schon et al. 2004). Klonisch et al. (2006) de scribed the mech a nisms of sea sonal changes in the testicular tis sue of the roe deer. The morpho-func tional con di tion of tes tes at the dif fer ent stages of a pop u la tion fluc tu a tion has been stud ied in suf fi ciently for voles, in which re - pro duc tion be gins in the spring and which ex pe - ri ence high mor tal ity rates in au tumn. De gen er a tive pro cesses in the seminiferous tu - bules cause the for ma tion of patho log i cal and atyp i cal poly mor phic sper ma to zoa with re duced fer til iz ing abil ity (Roosen-Runge 1977). There are, how ever, only frag men tary data on the morphofunctional con di tion of the tes tes and sperm mor phol ogy in murid ro dents at dif fer ent phases of a pop u la tion fluc tu a tion. Stud ies of the morpho-func tional con di tion of both the tes - tes and sper ma to zoa pro vide an op por tu nity to eval u ate in ter ac tions of gen er a tive po tency and fer til ity of males. We there fore con ducted a cytomorphological anal y sis of tes ti cles and sper - ma to zoa to eval u ate the gen er a tive po tency of the male bank voles at dif fer ent pop u la tion den - si ties. Ma te rial and meth ods Study area Sex u ally ma tured male the bank voles were caught on the Mid dle Urals (57 21 N, E) in June and July in years Based on a com plex ity of de mo graphic vari ables (pop u la tion den sity, sex ra tio and age struc ture, rate of sex ual ma tu rity, and breed ing in ten sity) 1990 and 1993 were re garded as low phases of the pop u la tion cy cle (2.45 voles/100 trap days), 1991 and 1994 as in crease phases (4.34 voles/100 trap days), and 1992 and 1995 as peak phases (9.31 voles/100 trap days).

3 Density-dependent regulation mechanisms 3 Pro ce dures Re moval of an i mals from the pop u la tion in the Sverdlovsk Prov ince was in ac cor dance with the rec om men - da tions of the Di rec tor ate of the Fed eral Ser vice for Con trol in the Field of Nat u ral Re source Use. The num bers of trapped an i mals were as fol lows: in July 1990 n = 5 and in July 1993 n = 7, in June 1991 n = 14 and in June 1994 n = 22, in June 1992 n = 12 and in June 1995 n = 23. A to tal of 83 bank voles were lived trapped. The an i mals were killed by cer vi cal dis lo ca tion and the tes tes re moved and fixed in 10% for ma lin, em bed ded in par af fin, sec tioned at 5 mi cron (µm), and stained with hematoxylin-eosin. Both right and left tes tes were sub jected to mor pho log i cal analysis (n = 166). Cross-sec tions of the tes tes were stud ied with the fol - low ing re sults: mor pho log i cal changes of the Leydig cells, which pro duce tes tos ter one; mor pho log i cal con di tion of the wall of the seminiferous tu bules, which in cludes one of the com po nents of the blood-tes tes bar rier (Sertoli cells); de - struc tion of the wall of the seminiferous tu bule that may lead to the breach of pen et ra bility of the blood-tes tis bar rier and to death of Sertoli cells, which bring about the phagocytic and hor monal-reg u la tory func tions of the tes tes; de gen er a tion of spermatogenic cells; spermatogenous ep i - the lium (dis or ga ni za tion of spermatogenous cells brings about the breach of spermatogenesis); seminiferous tu bules (an ab sence of spermatogenous cells leads to the at ro phy of seminiferous tu bules). Sper ma to zoa ex tracted from an epididymis of each an i - mal were placed on the ob ject-plate and mixed with a drop of phys i o log i cal so lu tion. The smears of sper ma to zoa were stained with azur-eosine and the prep a ra tion ex am ined with a use of light mi cro scope (at mag ni fi ca tion 1350), with oil im mer sion. We ex am ined 100 sper ma to zoa within a smear from an each an i mal. A SIAMS Photolab (Pat tern Ap proval Cer tif i cate of Mea sur ing In stru ments Ru.C AN 18352, Ekaterinburg, Rus sia) was used to mea sure the fol low ing pa ram e ters of sper ma to zoa: area of the nu cleus (µm 2 ), area of the acrosome (µm 2 ), length of the mid dle part of the tail (µm), length of the main part of the tail (µm). Patho log i cal forms of sper ma to zoa were reg is - tered: eg, those with mor pho log i cal de fects of the head, neck, and tail. Sta tis ti cal pro cess ing of the data was made with the use of monofactorial, nonparametrics dis perse anal y sis (Kruskal-Wallis ANOVA). Re sults Body mass Dur ing the low phase, the mean body mass of adult males was 19.3 g, dur ing the in crease phase 24.5 g, and at the peak 27.4 g. Body mass of males dif fered sig nif i cantly be tween the in - crease peak and in crease low (one-di men - sional dis per sion anal y sis, p < 0.001). Mor pho log i cal anal y sis of tes tis De struc tive changes in the tes tes of ma ture males are listed in Ta ble 1. The per cent ages of an i mals with de struc tive change dur ing dif fer - ent phases of a pop u la tion fluc tu a tion are pre - sented in Fig. 1. De struc tion of the wall in seminiferous tu bules (Fig. 2) re sulted in a par - tial death of the Sertoli cells that pro mote an in - ten si fi ca tion of de struc tive pro cesses in spermatogenous ep i the lium. The grad ual spread ing of the de gen er a tive changes within testicular tis sue at the peak re sulted in the dif - fuse spermatogenesis in sol i tary seminiferous tu bules and karyopycnosis of the spermatogenous cells (Fig. 3). Dis tur bance of spermatogenesis and death of spermatogenous cells also causes at ro phy of some seminiferous tu bules. At the pop u la tion peak, 60 70% of the Ta ble 1. The types of de struc tive changes in the testical tis sue. Morphofunctional in di ces of a tes tis De struc tive changes The type of changes Interstitial tis sue (the Leydig cells)-androgenesis Cy to plasm oxyphility I Karyolysis II The seminiferous tubule wall, the Sertoli cells structural The destruction of the seminiferous tubule III components of the blood-testicular barrier wall and Sertoli cells Spermatogenous epittelium- the process of the sexual cells Karyopycnosis IV development Karyolysis V Seminiferous tu bule the spermatogenesis ac tiv ity in dex Diffuse aspermatogenesis VI At ro phy tu bule VII

4 4 V. Mamina and O. Zhigalsky Frequency of occurrence (%) ,2 2,3 1,2 2,3 1,2 1,2 I II III IV V VI VII The type of destructive changes 2,3 Increase - 1 Peak - 2 Low - 3 Fig. 1. Per cent age fre quency of oc cur rence of de struc tive changes of dif fer ent types in the tes ti cles of bank voles in re la tion to the phase of pop u la tion cycle. Num bers above bars: a phases of a cy cle ac cord ing to a leg end. Dis per sion anal y ses were used to sig nif i cant dif fer ences be tween phases (1 2, 1 3, 2 3). Sta tis ti cal sig nif i cance was de ter mined at p < See Ta ble 1 for de - scrip tions of changes I VII. (a) (b) Fig. 2. Tes tis of a bank vole. (a) a cross-sec tion of a seminiferous tu bules: 1 a seminiferous tu bule with nor mal spermatogenesis; 2, 3 At ro phy of seminiferous tu bules; Pwt a wall of seminiferous tu bules with out changes; Atw the de - struc tion of seminiferous tu bules wall; (b) a frag ment of seminiferous tu bules 1, there are all types spermatogenic cells (Sg spermatogonii, Sc spermatocyte, Sd spermatid, Sz sper ma to zoa). Strictly con cen tric lay ers lo cate cells.

5 Density-dependent regulation mechanisms 5 (a) (b) Fig. 3. (a) a cross sec tion of seminiferous tu bules with dif fuse aspermatogenesis (desorganization of lay ers, a cha otic ar - range ment spermatogenic cells), (b) a frag ment of seminiferous tu bules, karyopycnosis of spermatogenic cells. Fig. 4. At ro phy of seminiferous tu bules (At), there are only Sertoli cells (St) and spermatogonii (Sg). males dis played seminiferous tu bules with dif - fuse aspermatogenesis and com plete at ro phy (Fig. 4). Spermiogram anal y sis Sper ma to zoa con sist of a head, which in - cludes a nu cleus, acrosome, and the mid dle and main parts of the tail. Study of smear prep a ra - tions re vealed vari a tion of the fol low ing pa ram e - ters of the sper ma to zoa: (1) size of nu cleus, from 5 to 25 µm 2 ; (2) size of an acrosome, from 4 to 17 µm 2 (Fig. 5); (3) length of the mid dle part of the tail, from 17 to 31 µm; and (4) length of the main part of the tail, from 41.0 to 70.5 µm. Acrosome size, as well as length of the mid dle and main parts of the tail ap pears to be in de pend ent from the phase of pop u la tion fluc tu a tion (Ta ble 2). Vari a tion in the nu cleus of sper ma to zoon were re lated to phase, with val ues of 18.0 µm 2 dur ing the in crease, 13.6 µm 2 at the peak, and 11.7 µm 2 dur ing the low phase (Ta ble 2). The high vari -

6 6 V. Mamina and O. Zhigalsky % of cells (a) (b) increase peak low Size (µm ) 2 Size (µm ) Fig. 5. Frequency of sper ma to zoa sizes: the nu cleus (a) and the acrosome (b) in a bank vole in dif fer ent phase of the pop u la tion size. Ta ble 2. Morphometrical in dexes of bank vole sper ma to zoa in dif fer ent phases of pop u la - tion cycle. The anal y sis of a vari ance has shown es sen tial dis tinc tions of a size of the nu - cleus sper ma to zoa be tween phases: In crease-peak and In crease-low (p < 0.001). MS b mean square be tween groups, MS w. mean square within groups intrasample vari ance. MS b df MS w. df F p The size of the nucleus < The size of the acrosome abil ity of the size of the head ap pears to be re - lated to dif fer ences in DNA con tent, which de ter mines the level of sper ma to zoa ma tu rity (Bouters et al. 1976). Dur ing the in crease phase of the cycle, males have sper ma to zoa nu clei with both loose and dense chromatin con den sa tion that sug gest dif fer ent lev els of ma tu rity. Spermiogram anal y sis showed a pres ence of patho log i cal forms of sper ma to zoa (the tail in the shape of a loop), but in only 10 15% of the males. At the peak phase of the cycle, most sper ma - to zoa had nu clei with dense chromatin pack ing that tes ti fied to a high level of ma tu rity. Fifty per cent of the males dis played up to 35% patho - log i cal epididymal prep a ra tions. Dur ing the low phase, the oligopyrene sper ma to zoa (with the size of nu clei from 5 to 9 µm 2 ) did not ex ceed 10 15%. Dis cus sion An im por tant bi o log i cal fea ture of cy clic pop - u la tion fluc tu a tions of voles is phase-re lated changes in body mass, with adults in high-den - sity phases 20 30% greater than those in low-den sity phases of a cy cle. This ob ser va tion, called the Chitty ef fect, is con sid ered to be a ubiq ui tous fea ture of cy clic pop u la tions. There is, how ever, no agree ment among bi ol o gists re - gard ing the causes of the Chitty ef fect. Oli (1999) pre sented a hy poth e sis to ex plain the Chitty ef fect, based on phase-re lated, dy namic al lo ca tion of en ergy be tween re pro duc tive and so matic ef fort. The ef fects of so cial stress on the im mune and re pro duc tive func tions of small mam mals have been well-doc u mented. In creased glucocorticoid lev els dis rupt and sup press en do crine sig nal ing in the male re pro duc tive axis (Hales and Payne 1989). Glucocorticoids have been des ig nated

7 Density-dependent regulation mechanisms 7 DENSITY OF POPULATION A TESTIS Endocrine part I, II, III Germinative part IV, V, VI, VII %? 20% 5-10% 20% 50%? 30% Spermatozoa morphology Spermatozoa quality 1 Olygopyrene Pathological ? 1% 20% 20% 10-15%? 30% 10-15% Fertilizing ability of spermatozoa Generative potency Fertility Fig. 6. Sche matic rep re sen ta tion of the mech a nisms during den sity-de pend ent changes in the tes ti cle sper ma to zoa fer til - ity sys tem in the dif fer ent phases of pop u la tion cycle: 1 in crease phase, 2 peak, 3 low phase; I, II, III, IV, V, VI, VII dif fer ent types of de struc tive changes (see Ta ble 1 for de scrip tions). stress hor mones be cause cir cu la tion lev els rise sharply in re sponse to stress, re sult ing in testicular in vo lu tion and a sig nif i cant drop in tes tos ter one se cre tion. These changes are ac - com pa nied by di min ished li bido and fer til ity. The changes in hor monal func tions caused by pop u la tion den sity is the con se quence of the pre - vi ous phase of a pop u la tion fluc tu a tion. At the peak of pop u la tion cycle dis rup tion of the neuroendocrine com plex is ob served. The neuroendocrine mech a nisms slow ing or stop - ping pop u la tions growth, are ini ti ated dur ing in - crease and are con nected with early dif fer en ti a tion of hy po thal a -

8 8 V. Mamina and O. Zhigalsky mus-hypophysis-ad re nal gland sys tems. Res to - ra tion of the mech a nisms pro mot ing pop u la tion growth dur ing the low phase are con nected with early dif fer en ti a tion of hy po thal a - mus-hypophysis-thy roid gland sys tems, leads to de creased thy roid hor mone pro duc tion and ac ti va tion of re pro duc tive func tions (Vladimirova et al. 2006). Thus, func tion ing of the go nad-ad re nal sys - tems may de fine re pro duc tive suc cess of a pop u - la tion. In our study, dur ing the pop u la tion in crease, re duc tion in androgenous func tions of the tes tes was ob served in about 50% of the males and at the peak up to 90% of the males (karyolysis of the Leydig cells, wall de struc tion in the seminiferous tu bules). The re duc tion of the androgenous func tion ap par ently was due to in hi bi tion of go nad o tro pic hor mones through an en hance ment of adrenocortical ac tiv ity. Sup - pres sion of androgenesis may lead to ei ther in hi - bi tion or to com plete ces sa tion of spermatogenesis in the seminiferous tu bules. Re duc tion of gen er a tive ac tiv ity of the tes tes is most likely con di tioned by both sup pres sion of spermatogenesis and de gen er a tion of the spermatogenous cells. Dur ing the in crease phase, changes in spermatogenesis (dif fuse aspermatogenesis) and the at ro phy of seminiferous tu bules were ob served in 50 and 30% of the males re spec tively, and at the peak in 70 and 60% of the males. Dur ing the low phase, dif fuse spermatogenesis and at ro phy of seminiferous tu bules was ob served in 50 and 30% of the males re spec tively. Spon ta ne ous death of cer tain classes of germ cells is a con stant fea ture of nor mal spermatogenesis in a va ri ety of mam ma lian spe - cies, in clud ing hu mans (Kerr 1992, Kimura et al. 2003). This in ter nal death pro gram leads to typ i - cal mor pho log i cal and bio chem i cal changes that have been termed apoptosis. Germ cell apoptosis is un der en do crine, cell so cial as so ci a - tion, and ge netic con trol. With drawal of gon ado - tro pins and/or tes tos ter one ac cel er ates germ cell apoptosis (Woolveridge et al. 1999). Tes tos ter - one, sup plied by ma ture Leydig cells, is a nec es - sary re quire ment for spermatogenesis. De gen er a tion of spermatogenous cells in creases due to de struc tion of the wall of seminiferous tu - bules and the death of the Sertoli cells, which are re spon si ble for nour ish ing, phagocytosis, and en do crine (es tro gen) func tions (Sharpe et al. 2003). Dur ing the low phase, changes in es tro - gen ac tiv ity were ob served in 40% of the males in our study. The seminiferous tu bule walls and Sertoli cells are known to be struc tural com po - nents of the blood-testicular bar rier (Rus sell et al. 1989). Breach of per me abil ity of the blood-testicular bar rier can ini ti ate the au to im - mune pro cesses in the tes tes, which pro mote be - gin ning of the de struc tive changes in spermatogenous ep i the lium. The low phase of pop u la tion den sity, as a rule, fol lows the peak phase, when the an i mal s growth and de vel op - ment take place at high pop u la tion den si ties and un der the high phys i o log i cal stress. De - struc tive changes in the testicular tis sue,which we ob served dur ing the low phase, most likely were a re sponse to en do crine con di tions dur ing em bry onic de vel op ment, which in turn was de - ter mined by the phys i o log i cal sta tus of the fe - male dur ing preg nancy (Tkachev 1980). De gen er a tion of spermatogenous cells in the tes tes of ten gives rise to poly mor phic sper ma to - zoon. De gen er a tive changes in the ger mi nal tis - sue cause elim i na tion and de gen er a tion of the chro mo somes that re sults in for ma tion of the oligopyrene sper ma to zoa (Roosen-Runge 1977), ie those with in suf fi cient chromatin (with a small head). Dur ing the in crease phase re pro - duc tive po ten tial of males de creases due to patho log i cal sper ma to zoa. At the peak, we ex - pected de creased re pro duc tive po ten tial as the num ber of patho log i cal sper ma to zoa dou bled, and 20% of the sper ma to zoa have low amounts of chromatin. Dur ing the low phase, re pro duc - tive po ten tial re mains low, be cause of the pres - ence of olygopyrene and patho log i cal sper ma to zoa. Patho log i cal forms of the sper ma to zoa are also the re sult of de gen er a tive changes in the spermatogenous cells. Patho log i cal sper ma to zoa with loop-shaped tails lose their mo bil ity, thus re duc ing the abil ity to fer til ize eggs. As a rule, oligopyrene sper ma to zoa do not take part in fer - til iza tion (Roosen-Runge 1977). Pres ences of hy - per- and oligopyrene sper ma to zoa re duce fer til ity of males (Beatty 1970). De gen er a tive

9 Density-dependent regulation mechanisms 9 changes in testicular en do crine func tion re sult in the re duc tion of an dro gen prod ucts, which af - fects fer til iza tion abil ity of sper ma to zoa. Menkveld et al. (2003) found a pos i tive cor re la - tion be tween the num bers of patho log i cal, im mo - bile forms of sper ma to zoa and the num ber of dam aged acrosomes. We ob served acrosome vac - u ol iza tion in a few sper ma to zoa, which also could fa vour the re duc tion of their fer til iza tion abil ity. We ob served the most marked de struc - tive changes in the tes tes and sper ma to zoa mor - phol ogy at the peak phase of a pop u la tion fluc tu a tion. The cytomorphological anal y sis of the tes tes and sper ma to zoa in the bank vole al - lowed us to de scribe the mech a nism of changes go ing in the tes ti cle sper ma to zoa gen er a tive po tency of an an i mal sys tem dur ing the dif fer - ent phases of a pop u la tion fluc tu a tion. Change in pop u la tion den sity led to a de crease in to tal num ber of sper ma to zoa and an in crease in the fre quency of patho log i cal and oligopyrene sper - ma to zoa. The morphofunctional con di tion of sper ma to zoa de ter mines their abil ity to fer til ize an egg, which af fects re pro duc tive po ten tial of males. Changes in the level of de struc tion of the testicular tis sue and in the fre quency of patho - log i cal and oligopyrene sper ma to zoa, as well as in the fer til iz ing abil ity of sper ma to zoa and the fer til ity of males as a whole, are one of the reg u - la tory mech a nisms in the den sity fer til ity sys tem. A sim i lar scheme of in ter ac tions be - tween the pop u la tion den sity and re pro duc tive abil ity of males may char ac ter ize the pop u la tion fluc tu a tions of other murid spe cies. Neurohumoral pro cesses are in volved in the de vel op ment of distrophic pro cesses in the go - nads of mam mals. In trin sic fac tors may re sult in de struc tive changes both in the tes tes and ova - ries that are ca pa ble of pro vok ing a de lay in spermatogenesis and follicular mat u ra tion in the young an i mals, which in turn pre vent pop u - la tion growth. Ref er ences Aguilera G. and Rabadan-Diehl C Vasopressinergic reg u la tion of the hy po tha lamic pi tu itary ad re nal axis: Im pli ca tion for stress ad ap ta tion. Regulatory Peptides 96: Beatty R. A The ge net ics of the mam ma lian ga mete. Bi o log i cal Re view 45: Bernshtein A. D., Zhigalsky O. A. and Panina T. V Multi-an nual fluc tu a tions in the size of a pop u la tion of the bank vole in Eu ro pean part of the So viet Un ion. Acta Theriologica 34: Bj rnstad O. N., Stenseth N. C., Saiton T. and Lingjaerde O. C Map ping the re gional tran si tion to cyclicity in Clethrionomys rufocanus: spe cial den si ties and func - tional data anal y sis. 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